Very disappointing considering what it could have been. I can't say that this was a really bad movie, but I have to say that I was disappointed considering the quality of the cast. Sad to say, I think the story was not the problem with the movie (obviously, it has been done before and done much better), but rather the actors and acting were.
I must take exception with some of the other reviewers; I think John Malkovich's performance was dismal. He shouldn't have been in this movie at all. Gerard Depardieu was not given anything to work with; perhaps he should have had Malkovich's role. Jeremy Irons, obviously a fine actor, chose (if he in fact chose it himself) a completely wrong attitude for the role. The three of them seemed totally uninvolved in the story, and had virtually no emotion at all. Even when his son was killed, Malkovich merely goes through the motions, not displaying the least amount of real passion. His vow of vengeance rings completely hollow.
The movie is saved from being bad by three performances -- actually four, but by three actors. DiCaprio, in the dual role, rises above the dismal acting of most of his supporting cast to give two solid performances. But the movie is completely stolen by Gabriel Bryne, who gives the best performance I've ever seen him give (indeed, I think it might have been award-winning in a better film), and Anne Parillaud, as the mother forced to choose between the son she raised who is a monster and the one she abandoned who is good. Their characters were so much more interesting that I found myself wishing the movie were about them, and the whole iron mask thing was used as window dressing.