I don't even know where to start, so I'll just say this: Go watch it if you haven't!!!
From the casting to the story to the ost, everything was absolutely amazing. So much so that I want to rewatch it already.

Firstly the cast: Byeon Wooseok and Kim Hyeyoon have such amazing chemistry, which just made it easy to get super invested in their story. I was all in after the pilot already and that is something not many (if any) dramas have ever achieved. Aside from the two leads, I also liked all the side characters a lot. For once, I didn't fall victim to the second lead syndrome because Ryu Seonjae is just THAT GUY. Somehow Byeon Wooseok manages to convincingly play both a teenager and a 30-something year old, despite knowing he is the latter in real life. And then there's Kim Hyeyoon who just made me feel all of the same emotions as her, her acting is just so great. It constantly made me wonder what I would do in her situation and often times I could so viscerally feel her desperation.

As for the story: I really wasn't sure about how they would carry out the plot and still keep it engaging but they totally exceeded my expecations. The first eight episodes (roughly) set up lots of background info but without feeling too overwhelming or boring. In the second half of the drama is where shit really goes down, though that is not to say that the first half is unnecessary or not enjoyable. On the contrary, it sets up all the characters and their backstories nicely which really gave me the time to get invested and attached to them. There is not a single episode that I can think of that was even the slightest bit boring to me.

Lastly the ost: when I say that this is my favorite ost of all, I'm not exaggerating at all. All of the songs are amazing but I just love it when the one of the leads sings one of the songs because it just adds so much to the drama for me. Considering Seonjae is an idol, it only made sense for Wooseok to sing an ost and he delivered!!! I've been listeing to Sudden Shower, Spring Snow and of course A Day (where my other atinys at?) so much and I can't get enough. Every time I heard Jonghos voice, I knew I was about to cry and I can't even be mad because it's so beautiful. All of the songs were just so perfect in conveying the mood of the scenes.

So to end it I just want to say: this is an amazing drama and I would love to see more from this production crew, they really did a great job at bringing an awesome cast together with a great story and ost.
When I tell you I cried at least once watching most of the episodes, I'm being entirely honest. This drama was an emotional rollercoaster, in a good way.
Also watching this as it was still airing was certainly good test of my patience because I could barely wait for the next two episodes at the start of every week. Can't wait to rewatch this and be able to binge it in a few days!