On the one part, is an usual, iconic Asian tragic love story. On the other side, it's one of the most fulfilled aesthetically and emotionally of the kind.
One thing that I love of this drama is the poetic power of the scenes, the combination of music-view and a perfect symbolic charge of many moments (as the very first scene, one of the best introductions to a drama I've seen, where Rain's ans Shi Min Ah's characters are laying surrounded by the nature but in different scenery and a wonderful play of light and a deeply touching song make you feel the complicated fate laying in the atmosphere; or, for example, their arms protruding through an open window, capable to transmit the sensation of wind even to a spectator). The story itself, as I've said, is really quite predictable but it's emotional tension is at the point (I don't speak only about the central love story but also about brothers relationship, especially then one of them is disabled).
The acting of the main characters is wonderful. To tell the truth, I'm not a fan of Rain as his characters usually look silly to me but here I can give him 10 points without any doubt. He's complicated, mysterious, attractive, charismatic and moving in this drama. An absolutely flawless acting. Then he feels pain, you can really believe in it (in other dramas I've seen with this actor, unfortunately, I felt a lack of moving strength). It's also my favorite role of Shin Min Ah. She's really very convincing and charming here.
The soundtrack is also absolutely perfect to the drama and one of the best I've ever heard in an Asian drama. Also, somehow the drama has a certain integrity, a strong and captivating atmosphere. It's really worth to watch, although the rating is suspiciously low :O