It rarely happens that as soon as I finish watching a drama, I want to watch it all over again, but this drama managed to get me hooked on it since the first scenes. Sometimes I got the impression that some interactions were a bit forced and not completely natural, but that didn't spoil my perception of the plot or the characters. I loved how the writers dealt with such a thorny argument as mental disabilities with such delicacy, giving the audience so much hope that with the right combination of medicine and love it is possible to lead a fairly normal life (even if it depends on the gravity of the illness, of course). The interaction among all of the characters was incredible and the main leads are so in synch with each other to give the impression of actually being dating IRL.
And the music... I can't even begin to describe how good the OST was. Each song choice was perfect for the moment it accompanied and made me feel like buying the soundtrack CD.
To sum it up, this is a drama I strongly recommend.
It was fairly good until it reached the end... and then it kinda got ruined. I think that the authors wanted it to end with an unexpected plot twist, but failed to deliver it in the right way, because the ending just feels kinda rushed really. (It's not entirely bad... just leaves you with a sort of WTF-just-happened kind of feeling).
Kudos to BoA, anyway, in her first big role on screen. Her acting was good, considering she's mainly a singer.
All of the main characters appeared to be somewhat lacking in one way or another, but at least some of them managed to improve themselves.