Story: This story is so good and original. I was absolutely entertained all throughout this drama. WARNING: If you watch this drama you will become addicted and won't have time for anything else but this drama until you finish.
Haha so make sure you don't have any tests or finals you need to study for while watching this drama.
I loved it. I hardly rate any drama's ten, and I rate this one a ten. This is definitely one of my favorites.

Acting/Cast: Ji Sung did an absolutely amazing job with his character. Everytime he changed his personality he really changed, I could hardly recognize him. He was also very moving, I cried so many times. He was great.
As for the rest of the cast, they did a good job too.
The only reason I give this category a 9.5 is because of Hwang Jun-eum's crying.... When you watch it you will see what I mean.

Music: Very good music

Rewatch Value: The only reason I give this a 9.5 is because I don't really rewatch dramas. But if I had to, I would probably rewatch this one.