I thought this drama was very well-done and highly entertaining. Was it perfect? Of course not. But I don't get the hate I read in some reviews and in the Viki Timed Comments while I was watching. It is surprising to me that this drama provoked such extreme reactions, with people either calling it the best of the best (which it isn't) or hating it to bits (which I don't think it deserves). Personally, I lean more towards the positive, although I found some weaknesses, that I will describe below.

The time travel

I happen to like dramas with second and third chances, where the character travels to the past trying to change things and, if it's done properly, I don't mind at all seeing the same things happening in different ways. It is exciting to think about the implications that changing one single thing can have to the future of all those involved.
I loved "Operation proposal" with Yoo Seung-ho, I also liked "Tomorrow with You", I hated "Manhole" with a passion. I loved "Twinkling Watermelon", except for the finale. I felt really let down and cheated of my happy ending. After all this desperate struggle of the son to save his future father, the accident happens anyway - although the outcome is significantly better. I felt like the writers went for the politically correct ending, with all its absurdities (he gets an executive job, of all things, in a company selling music instruments where everyone learns or at least understands sign language, just to accomodate one person who is not even close to them).
I was afraid of this here as well. I was afraid that they would have her end up paralyzed anyway, but with the love of her man, saying "there's nothing wrong in being disabled, it's just as good as not being disabled, you can be happy anyway". I've heard the argument that her ending disabled anyway would have been more realistic? Realistic? What on earth is a realistic outcome in a time-travel k-drama? Does time-travel exist and does it have real rules, realistic or otherwise? What are our criteria for calling something "realistic" in that context? Thankfully they didn't go that route.
Although I thought that overall "Watermelon" was a better drama, this one had its own charm.
I liked that the female lead does not travel to the same moment in time, and also that her visits are not always in chronological order. This gives variety and freshness, and helps us explore different moments of the characters' lives.
I didn't like that the reason and rules of time travel were not explained properly (was the grandma the Heavenly Granny after all, as was hinted at the end?) Just because the clock showed 3 o' clock and the next time 2 o' cloth, she assumed that there were three tries? That's random. But then, time travel dramas usually are a bit vague about that part, you're expected to just suspend disbelief.

Was the main lead a "loser"?

Someone said that it was unnecessary that the male lead is revealed to have liked her since the beginning. That he has no reason to like her. Yes, it wasn't indispensable, yes he could have grown to like her little by little.
But that revelation makes it much less one-sided, it made for a nice twist to see everything from his point of view. And, more importantly, given her crazy antics the first time she travelled, I don't know whether it would have been reasonable to put up with her if he didn't like her beforehand. He would have brushed her off as a weirdo. I know I would have! And, okay, let's be real, teenagers don't need very deep reasons to like someone. Consider that he went to an all-boys' school and wasn't very exposed to females, dedicated to his sport as he was. And at that age where hormones are raging. He saw a pretty girl with a radiant smile and he was smitten. At that point, it was just a generic liking, someone you would wish to meet again and make an acquaintance with, not the great love of his life, for whom he would be ready to give up his own - this came later, in a much more organic way.
There was the criticism that the main lead only thinks about his love and not enough about his career. He forgets about swimming too easily only because of her. Yes and no. The way I understood it is that while he was young he did like swimming and it was his life, but he was also encouraged a lot by his father, and he was happy to be able to excel somewhere. The time he had taken away from the sport led him to explore other interests as well. He did get interested in singing after that first time he got on the stage. He did write a song (inspired by his love, yes, but aren't 95% of songs love songs anyway?) Let's not forget that we're talking about 18 or 19-year-olds. They can change interests. I think that he really didn't want to go the the US for rehabilitation. First he was afraid that he wouldn't make it anyway and he would have raised his and his father's hopes for nothing. I didn't feel that was an excuse, it was a very realistic fear he had. Yes, not going to the US had a lot to do with his love, we know that. But I don't think it's the whole story. Not everyone goes on to do what they did when they were very young. So I don't find this a flaw. Otherwise, yes, he is ocncentrated on her a lot, but aren't we all like that when we are in love? Why call him a loser, as so many people did? Are you a loser if you are able to feel deep love and show it to your beloved? I know, she rejected him so he should move on. But we also know that in reality she liked him a lot. Her eyes often gave her away, her care for him etc... Why not assume that something within him sensed that, he felt that he was loved back, that while her lips told him to go away, she was unconsciously sending him messages to stay, and that's why he couldn't move on? It wasn't spelled out, but that's a factor to consider.
Was it unlikely that the male lead had no other relationships in every possible timeline, even in the one where he had never met her and didn't remember her? Of course it was. I agree with that. But we're watching a k-drama, and this is a k-drama trope. In k-drama world this is the normal thing. Being an idol, it's slightly more likely, as they are closely watched and fearful of scandals, but we know that real idols have relationships anyway, secretly.
The Viki comments consistently called him a loser. How can you call a man a loser when a woman goes through all of this only to save his life, even to the point of risking her own? Just because she doesn't show it and he does, he's a loser? What a shitty, old-fashioned mentality is this? She doesn't play hard-to-get because she's playing games, but becasue there are very important things at stake.

In defense of the female lead.

While I was watching at Viki, many Timed Comments were very critical (nasty I'd say) of the female lead, especially every time she rejected him. And here on MDL, at least two reviews criticized her for not using her time in the past to enjoy the company of her family, instead of making him her only pursuit. People, did you forget? The goal of this trip was not to have a cozy time with friends and family but to save a person's life! And especially once she got the information that the man dies because of her, it's absolutely normal that this desperate quest takes top priority. Moreover, she doesn't seem to have been the sort of person who didn't spend time with her family anyway, one who didn't appreciate the grandma while she had her with her mind intact. So there was no special need to do something extra to make up for lost time, because there was no lost time. That would have been an important thing to do if she had a weight on her conscience that she took her mom and grandma for granted, that she did not spend enough time with them or give them enough affection. But we were shown that they were a close-knit and loving family anyway, so this need would not have overriden the mission to save the life of an innocent person who died unjustly in the prime of his youth. That's all. I don't think I have to add more to this.

And what? they mock her for crying in every episode? How couldn't she cry? When her desperate attempts to achieve her mission fail, time and again, when she's chased by a murderer and she has to witness this young man's death not once but multiple times? Who wouldn't cry, in real life? Then others criticize her for being too bubbly. Make up your mind, is she too sad or too bubbly? There's no satisfying people. As for me, I found the bubbliness forced in some scenes where she's pretending to be carefree and wants to hide something. She's very obvious. The actress is actually quite good but the character was not necessarily a good actress and they want to show us, the audience, that she's lying, doing it in a less than subtle way.

Did Im Sol always make good choices? Of course not. I thought it was careless of her, the last time, to assume that because the man hasn't met her, the killer is no longer a problem. She should at least search the news of the year 2008-09 to see whether he committed some crime and what happened to him. Because Ryu Seon Jae's fate of being killed by that man did not depend on his connection with Im Sol, as was shown at the end, where the criminal is about to kill him without even knowing who he is.
But if we found ourselves in that situation, would you and I make perfect choices every single time? Probably not. Right? The audience sometimes seems to think that the character knows what we the viewers know - what other characters are thinking or doing, where the killer is and so on. Of course this is not the case. The poor girl has to wing it, to do all she can with the available information she has, and with the chances she is given. She is bound to make mistakes. It's also for the sake of drama. The "hero's quest". Drama is made by overcoming obstacles. If there were no obstacles and she solved everything the first time around, the drama would be over in five episodes!
I kept reading "why is she breaking his heart, why is she so hard on him? Just date him and be happy". Those people seem to forget the main point of the travels her main goal, is to save his life.

My opinion on Im Sol's character? I found her very brave, she had the strength to stick to her main goal without wavering. Of course she longed to do what viewers wanted her to, of course she wanted to confess and throw herself into his arms. But if she did, she might have to face the trauma and heartbreak of his death again. After her first time travel, every time she went back with a very specific plan, and she stuck to it faithfully, no matter how much she was hurt, no matter how much he was hurt. And no, "telling him everything and figuring out solutions together" was not possible. We saw what happened the time before the last, when he understood her plan and said , "I don't care, let's love each other anyway and if I die I don't care". Which happened. He came back to the cliff to save her and got killed. So we were shown how this was not a good idea at all. She wasn't planning to blindly sacrifice herself. She had asked for backup from the policemen. She had a plan. Until he ruined it because he thought she was stupidly sacrificing her life.
This was a very smart heroine. Not perfect, but smart and loyal, and as much in love with him as much as he was with her, although she wasn't allowed to show it for most of the drama.

Secondary characters

Side characters are not well-developed and do not have any development arc, they don't mature, they don't change, they are one-dimensional. Yes, that's true. And, unfortunately this is too often true in k-drama (should I say most of the time?), it's one of the most frequent flaws. They are stock characters (the supportive friend, the goofy friend, the fat friend, the nagging but affectionate parent...) there only so that the leads have somebody to confide in, or just for comic relief. And it's a pity.
The second male lead is also not developed enough as a character. I agree with that as well. His family history and relationship with his father is only hinted at, and there's a sweet scene between them towards the end, which sort of mends things, but it could have been fleshed-out much more, since he does play an important role in the story at the end.
The killer. Yes, we would have needed some more of his motives, what his deal was. Not a lot of time spent on him, just a few hints on how and why he did what he did. Not that all serial killers have real reasons, but for dramatic balance it would have been nice.

Slapstick I hated

I was especially furious at how they used the ML's dad. Kim Won Hae is an amazing dramatic actor (as we saw in many many shows, for instance Good Manager and While You Were Sleeping). But since his role in Strong Woman Do Bok Soon in 2017, they have often been using him for heavy slapstick of the most vulgar kind. Since Strong Woman Do Bok Soon is another inspiration (together with Watermelon) for this show, they thought they would make supposedly fun scenes where he hurts himself and has to wash his soapy eyes with toilet water. I wanted to slap the writer for doing this. And yet people in the comments were saying it's so funny. I really don't understand these primitive people who can laugh with those things. Anyway, the actor being awesome as he is, he still manage to injet some humanity to his puppet-like character and show us his good heart and great love for his son. I thought this serious enough a flaw to take one star from my rating. There are more refined ways to make comedy, and we had some of those moments in the rest of the show.


Tropes.

Were there a lot of k-drama tropes? There were quite a few (way too many) "the female falls and the male grabs her to arrest her fall", there was also an accidental kiss if I'm not mistaken, there was the interruption of intimacy by an unexpected visitor and hiding into the closet, there was the botched proposal, the amusement park date and a few others. Although they were not overwhelmingly bothering, I wish they weren't there. And I was happy to see our old friend the white truck of death, for once in the role of the good guy. He deserved that break, after years of loyal service.

Product Placement

I was happy to not notice blatant product placement. Maybe the ice popsicles and a few others, but it was seamlessly woven into the narrative, so it didn't bother me. No female displaying all the range of a certain brand of cosmetics and suddenly starting to extol the virtues of some lotion out of nowhere. No Subway either. I didn't even notice the brand of the jewellery when Seon Jae went to buy the rings, so thanks show, for not being too pushy with those things.

The ending

One thing I was really grateful for is that the resolution didn't happen in the last five minutes, and we had a whole episode of happiness to enjoy, and we also were given the time to learn what happened to most of the other characters. That was super-refreshing. Some people found it "underwhelming" because the show didn't finish with a big dramatic splash. Personally I prefer them that way, rather than angst until the very end or, God forbid, an open ending, leaving you with all sorts of questions.

Music:

The Shower song was pretty good, I liked it, and there were a couple of other good ones. Not a score that will remain forever in my mind, but it served its purpose well.

All in all, I think it was a good drama. Not one of the ten best of all times, but a very good one nonetheless.