I was hooked by the first episode, but after watching a few more, I became more and more disappointed. First, here are the things I liked about the show:
1. The acting overall was pretty good and most of the main characters were charismatic and likable.
2. The pacing was fast and alternated scenes from different story lines so you didn't get bogged down too much.

Here are the things I didn't like:
1. The editing was insane, making you think that scenes were missing, and giving the episodes a rough, messy feeling. I thought at times that maybe it was intentional, but now I think it's just incompetence or a rushed production.
2. The sentimentality and overacting went too far, making me cringe. Kim Sang Kyung has given some really great performances, like in the movies Memories of Murder and On the Occasion of Remembering the Turning Gate. But whatever he was trying to do in Racket Boys, it didn't work. His performance was more appropriate for an annoying, weak-willed side character who ends up going into debt and causing problems for the main character, sort of like the Fredo Corleone of the badminton coaching world. But, instead, he’s the lead adult character. I can understand trying for something different to avoid the typical cliched lead performance, but this one didn’t work, resulting in an unlikeable, simpering character. Also, Kim Kang Hoon annoyed me with his sappy child actor stuff. I wish they had omitted this character from the story. The main boys as a group were likable, but they pushed the bromance sentimentality too far, and it was unrealistic and off-putting. I preferred the 2 girls, who seemed much more like real people because they weren't so over the top.
3. The portrayal of the sport failed. It was totally unrealistic, and the endless matches repeated over and over, with each one seeming to be the most important match of all time, only to be topped by an even more important match on the next episode, just made me numb to it all. I wish there were fewer matches and more training and actual progress and realistic pacing. And how many times do we need to see a slow-motion jumping overhead smash? Also, are we to believe that this tiny village in the boondocks has so many of the best badminton players in the entire country, as well as some of the greatest former players and coaches? They could have made a fun and much more realistic story about a normal group of badminton players.
4. The portrayal of humanity failed too. It seems strange to me that this show supposedly shows people at their very best, with friends, neighbors, and family helping each other and making sacrifices for each other. But this only seems to apply to the main characters in the story - the players who live in the house, their coaches, and the villagers. Outside of them, just about every other secondary character in the story is either a bully, a liar, a cheater, a greedy criminal, a coward, an arrogant asshole, or some other variation of villain. So the overall message is that there are a few good people in the middle of a big bad world full of horrible villains, and humanity is basically bad.

Despite all these flaws, I did power through to end, even though I was very close to dropping it. However, I had to increase the playback speed to 4x, and sometimes even 8x, to get through it, especially in the second half. If you have a high tolerance for sappy sentimentality, overacting, impossibly melodramatic sports heroics, and don't mind those things being repeatedly pushed in your face over and over again, then there's enough entertainment and likable characters beyond those things to hold your interest.