What to say? Outstanding. A masterpiece for its genre.
This is not a conventional love story, but an actual melodrama which will keep you glued to the screen till the last second.
The story is intense, intricate, and well developed in general (apart some minimal plot-holes). I liked the way it was structured in order to keep the spectators constantly in suspense, sipping the details episode after episode, but at the same time leading them to figure out the potential developments so that the attention as well as the expectation is always high. Tears (a lot) are assured. Some moments and key scenes are really cruel in their tragedy.
The Leads, simply awesome all the three. Seo In Guk confirms his talent as handsome and damned, able to recreate the dramatic traits of his character as expected, constantly immersed in his role. Jung So Min, perfectly suited for the role, well impersonating her character also phisically. Both of them showing an intense chemistry. Park Sung Woong, the gentle giant, who hides a dark past, loyal to her sister so far as to make extreme gestures. He's well completed by a brilliant Jang Young Nam, all along the story. Also the other support characters were developed and defined. Maybe the villains were on the verge of psychopathy, which could compromise their credibility.
Music, excellent. I confess that I fell in love for Lost by Lim Yeon.
Just a word to some who criticized, complaining about the lack of continuity, gigantic plot holes and not nice performances of some: you should watch and understand what you watched prior to saying nonsenses. Unless you were writing about other shows.
For the rest, keep the tissues ready and enjoy it.