it's not great drama, but it's interesting, caches you (it's rare for me not to skip), also there are interesting main and side characters.
before watching, from what I heard I thought ML was dark but likable, I like that kind anti heroes, but he is just bad person who never even understand it or apologized (especially to In Ho, he ruined his life). ML is just cruel jealous sociopath and it's unrealistic why FL tries so hard to understand/like him, when from the beginning she even didn't liked/wanted him, it was like she didn't liked him but made herself to fall for him.
Often anti heroes change/understand what they did bad, but in this case I couldn't see it, he is unredeemable though drama doesnt treats him that way, like it's normal (but he really does many unnormal and cruel things, which aren't cool at all)

on the other hand In Ho (Seo Kang Joon) was so great, so lovable person, FL was way more happy every time with him. he was more like ML, his character and story was more layered and reach, except of relationships to main characters he had more his own story line then ML (orphan, talanted pianist, sister issues, broken hand and carrier, criminals chasing after him, so warm relationships with many people, funny and also rude/tough when it's needed, back to playing piano, friendship/love to FL and many things), he had so complex life and problems...

I know many people think the end was disappointing and sad, but I think it was normal, how could she stay with that sociopath, I didn't liked that she wanted it and cried, why this drama romantizes really bad person???