Just to say: the Korean version of the movie is definitely better.
Old Boy is a manga that started with a great premise: a man was imprisoned for ten years by a mysterious person. He is then released and has to find out who did this to him and why.
It started really good. The ways he used to track the steps of the mysterious man were really innovative and cool. Nevertheless, while I was reading it, it seemed to me that the author started the story without an idea on how to end it, so he was just dragging the story ever further. It took 8 volumes to finish the manga, but it could've been done with half of it, or even less. That's why I gave a 4 to "Enjoyment": because at a certain point I wasn't able to bear reading it anymore. I couldn't wait for it to finish.
And the ending is terrible. The explanation for why everything happened is just terrible. That's why I'm positive that he didn't had the notion on the motifs of the "villain", and just made some crappy ending to wrap it up.
It had potential to be a great story, but in the end it was frustrating. Had he done the same way the Korean movie version, it would have been awesome.