At first, this show is really great! It's everything you want out of a rom-com k-drama. Likeable leads, nice flow, good acting, high production value. It's a bit heavy, but it's to make a point about the state of the insane educational environment currently prevalent in South Korea.
But, wow, when it starts to stray and put more focus on some of the other plotlines (around episode 10 I think it was), it gets real silly, real fast. It really loses all idea of what it's trying to say, and common sense flies entirely out the window in the process. Side characters become complete caricatures, situations become super forced. In particular, the first hour of the finale is wretched. It is, in all seriousness, truly insultingly bad. Literally everyone starts acting completely out of character, in an attempt to create as much random drama as possible when one can only assume they had completely run out of ideas and are trying to fill time until they can get to the proper wrap-up.
Anyway, overall it's worth watching, because it really is quite good for a while, but you might find yourself rolling your eyes a lot as it enters the stretch run, as I did.