Firstly, I must say that Ji Sung's role here is one of the most remarkable roles I've seen in a Korean drama. I was really impressed by his ability to transform himself in several totally different characters in the same drama and with such a credibility, and considering the stunning charisma he has, such an interesting character of a guy with a dissociative identity disorder gives him an amazing opportunity to reveal his artistic maturity. So, to tell the truth, Ji Sung's character here is one of the best kdramas characters I know, as the same actor is obviously well-known not only because of his really captivating beauty.
Therefore the story of such a character itself arouses interest but also there are some quite sensitive and cute elements and a huge amount of charming, playful moments what make the drama absolutely fulfilled. All in all, it's one of the best kdramas I've seen ;)