Watched it with my sister and it was one of the best Psychological Thrillers I have seen, in fact maybe the best. It was another piece of evidence of what a versatile actor Lee Dong-Wook is, having seen his performance in many other dramas. Im Si-Wan's performance was top of the line, although this was the first drama of his that I have watched. I do plan to find more of his dramas soon. Of course, kudos to the amazing acting of the rest of the cast. I can definitely watch this many times and not get bored.
We watched the episodes with so much dedication. However, I must admit that I am disappointed with the ending (not going to say what it was). I expected a totally different outcome.