One of the most controversial movies ever, Irréversible is possibly the most disturbing film I've ever seen.
The movie, which tells the story of a man avenging his brutally raped girlfriend, unfolds in reverse order. Was it told in chronological order, it would've gotten a 1-star rating. In fact, the only "great" thing in the movie is the way it progresses (or in this case, regresses).
The best aspect in the film is the editing, which is quite obvious due to the fact that it is told in back order.
The acting is very powerful, especially that of Monica Bellucci.
However, the worst thing in the movie is the fact that the two most disturbing scenes (Spoiler alert: A rape scene and a tour in a gay BDSM club) are stretched a lot. While each would've barely lasted 5 minutes in any other film, they were each stretched in this one for 12+ minutes. And that is just so that the director can disturb the viewer as much as possible. And he succeeds. In fact, both of these scenes are accompanied by music so powerful, it ends up disgusting you even more.
The best part in the story is the ending (or should I say, beginning?) which - I don't know why - reminded me of that of Stanley Kubrick's masterpiece 2001: A Space Odyssey (It should also be noted that a poster of this movie is seen in the film).
Finally, just for the sake of comparison, I would like to compare Irréversible to another extremely controversial movie from the past decade: Lars von Trier's Antichrist (2009). Both are foreign movies that feature extremely graphic violence and both tend to disgust the viewer more than anything else. However, while Antichrist is quite fascinating, re-watchable and thought-provoking, Irréversible ends up pushing the watcher away; it's a movie that you'd only want to experience once.

Bottom Line: Irréversible is one of the best movies that I wouldn't recommend.