I recently saw Old Boy and I Saw the Devil in sucession.
Old Boy was quite good. It was a bit silly and weird but never unrespectable. The cutting as well as the frames themselves were quite cool and somewhat innovative. Best of all the movie did not drown in its own weirdness, but it was a fun, fast-paced movie with some heart to it. 9/10
I Saw the Devil was a different sort of film. It was very different from standard western films and definitely a nice change. But the plot was not great and neither were the characters. BUT! It was very fun, both due to the plot and action, but also the raw unrelenting violence.