North American fans of Korean cinema will no doubt compare this movie to Oldboy for a couple reasons: #1, it's a film about revenge, made in Korea (their directors seem to know what they're doing when it comes to this sub-genre of thrillers) and #2, it stars Min-sik Choi, who was the main protagonist (or antagonist depending on how you want to look at it) of Oldboy. However, that's where the similarities end.
Minor Spoilers ahead: "I Saw the Devil" is about a man (Kim Soo-hyeon) whose fiance is murdered by a serial killer. The next 2 hours are spent with him hunting and attacking (over and over, in very creative and gruesome ways) the man who killed his fiance, Kyung-chul (played by the masterful Min-sik Choi). There's no real twist (like the big one in Oldboy that causes everyone I've ever known that watched it to say "holy ****!"), you're not trying to figure anything out, this is the plot and it's laid out for you very early on, though there are times throughout the film where you wonder who will wind up winning out in end as each man gains and loses the upper hand several times (and there is a clear winner).
Critics of the film feel that the movie is excessive in the violence, and they feel the choices made by the characters are quite questionable (especially at the end), but essentially it delivers a similar message that other Korean revenge films have: Revenge isn't satisfying. If you're a fan of blood and creative violence in cinema however, you'll like this, but it will make even the most hardened of gore fans squirm (the Achilles tendon scene, the knife in the hand scene, the cab fight, etc). This is Korean cinema at its least subtle... violent with a touch of very dark humour, just the way I like it.