It seems unfair to compare a small book with the sumptuous, intricate and extraordinary piece of filmmaking that was Park Chan-wook's Oldboy, but I don't feel like there's any other choice. The film towers over its source material and is so much more interesting and powerful than at least the first 10 chapters of the series.
The story, for a workaday pulp manga, seems like a relatively original idea, but of course it bogs down in traditional, solitary masculinity. A hyper-macho mega-manly man is out for revenge. He fucks a waitress, works at a sweaty, manly job to earn money for his revenge plot, and poses repeatedly for a low-angle viewer to appreciate his six-pack and laughably big guns. Which makes the film performance by Choi Min-sik that much more admirable, in that it had depth, weirdness, and vulnerability that wasn't part of the original text.
After getting through book 1, I'm not particularly inclined to hunt down the subsequent books in the series.