Brilliant and disgustingly so. Min-sik Choi did a wonderful performance with his character. The transformation from drunken lout to tormented soul was excellent then add into that moments of insanity. I'm sure playing a man who has been imprisoned in solitary confinement for 15 years was hard but I felt he pulled it off enough to make it believable.
The story line was great and had me just staring like a slack jawed idiot at times with scenes like when he gets to the office building and the teeth then afterwards (that's as much as I can mention without giving anything away) but the ending was was what made the movie. Not only finding out why he was imprisoned but that little extra bit of torment was what made it disgustingly brilliant. I'd highly recommend it but not if you're squeamish. Not that it's overly violent, blood thirsty (seen worse) or that you actually get to see the acts of violence but it's the knowing that makes you cringe.
10/10