The first in the vengeance trilogy by Park-Chan Wook, see my oldboy review the second film in the themed trilogy
here. By themed trilogy I mean all the films are about vengeance but there is no connecting story. Also note these films are in korean, so you'll be watching them dubbed.
First off this is considerably more tense then oldboy. We have what starts out as a story of a guy who is both deaf and mute wanting to donate his kidney for his sister. He then finds out he can't donate his kidney due to blood type. So he is obviously looking for a way for to find a kidney for his sister. This starts off a series of events, dealing with a variety of themes. First we take a look at the criminal world, how good intentions plus desperation can eventually lead people to very bad decisions. One thing I must say about both oldboy and Mr vengeance is just how real the characters feel. Oldboy focuses solely on one character and the story is very interesting in its own right. Mr vengeance actually deals with the fate of two people, there stories are intertwined and you can sympathise with both of them alot. This is both exciting and slightly disturbing. I found both of these films make you really question how thin a line there is between good and evil. The 'evil' acts of vengeance in this film would be considered perfectly normal reactions to the situation, which really just makes you realise the potential of a man on the edge.
I will say as great as this film it is definitely not for everyone. I would suggest not watching this with your girlfriend, I would say most girls who aren't into cult cinema will most likely find this uncomfortable and disturbing. If you are just a fan of commercial films and don't like giving things that are different I would say this is also best avoided. If it does interest you I would definitely recommend Oldboy over this as it isn't anywhere near as unsettling, although it still has a lot going on in it.
This is also a film you will need to pay attention as Nonny mentioned elsewhere there is little to no dialogue in this whole film. The deaf mute guy does not speak (obviously, but he also doesn't think much either), the other character (amazing actor) has no one to talk to for the most part. A lot of the film is given in visual cues, you defintely need to give this your full attention or things will go right over your head. This film isn't anywhere near as violent as Oldboy, if memory serves (I'll watch lady vengeance next and if time i'll rewatch oldboy), but it does have its fair share of moments.
Which film is better is something taht isn't clear Oldboy is certainly more accessible and I would recmmend it over this film but that isn't to say that its significantly better, its just easier to understand. The realism of the characters in this and the camera work in this film (yep i'm mentioning it again, not being pretentious i'm just shocked at the quality of korean cinema), is just amazing. You can see the fear in there eyes and the people act exactly as you would in that situation, I honestly have to say I've very rarely watched a film and got hat from it, but this film hits the nail on the head.
If you enjoyed Oldboy definitely give this film a shot.