This comment is meant mainly as a warning to the people who might be attracted to the title by its (temporarily)high user rating which I find frankly puzzling. The reasons why I didn't like this title are following:
1. The directer must have had some doubts whether to make a Jackie-Chan-type of a flick or a dark Oedipian tragedy. As a result, in terms of genre, the film falls between two stools, as the tragic and comic elements clash and cancel out each other rather than make a harmonious whole.
2. The characters' motives and behaviors are incoherent and unconvincing. Psychological truth and logic are sadly missing.
3. Absurd casting. I don't blame the actors, for it is a hard thing to create a convincing character by acting alone, if there is scarce logic in the script. However, why is there an apparent age difference of about 15 years between the leading two actors, whose ages in the film can't differ by more than 3-4?
4. To me the film was poor entertainment primarily because of point 2. If you can't find a character you could sympathize with it is hard to follow the story with interest. When you finally learn the reason of what happened to the main protagonist, it turns out to make no sense.
5. Some films apart from being entertaining are also thought-provoking. Having seen this film, I began to wonder whether the thought the director tried to provoke was not that incestuous relationships could be perfectly wholesome and delightful. I cannot put any other construction on the ending.
6. The film is rife with totally unnecessary violence. Violence in a film (and elsewhere) is a good thing, if it serves an important and worthy purpose. Purposes can be different and I don't want to enter into this broad subject. Let me just say I don't object to violence in such films as "Saving Private Ryan", "The Passion of the Christ" or "The Pulp Fiction". In "Oldboy" the scenes of torture and suffering are prolonged and graphic (or aural). What for? I do not know. Personally, I don't derive any satisfaction from watching teeth being extracted with a hammer or hear a man cut off his tongue with scissors and then see him choking on his own blood etc. etc.
7. The film reminds me a little of Japanese porno mangas in its fixation on incest and young Asian girls' panties, urinating and the like. It appears there is a minority who actually enjoy this kind of thing. If you're one of them, you might find this film enjoyable.
In short, I do not recommend this film either as entertainment or "food for thought". Where it isn't silly, it is disgusting. Don't waste your time.