Based on cinematography, direction, acting, and overall structure alone, this is a masterpiece. But I will never watch it again. Its depictions are so realistic, gritty, disturbing, I can hardly bear to think about it today. This is literally the most disturbing movie I've ever seen, including Martyrs. There were times when I had to look away for minutes at a time.
The first twenty minutes or so were hard to watch, not only because of some of the unnerving sights, but because of the camera flip-flopping all over the place. At one point I had to look away for a minute or so just to regain my sense of equilibrium. And throughout the movie the camera does some roller-coaster activity that sometimes heightens the calamity, and sometimes just makes your stomach lurch.
The best part about Irreversible is the philosophy behind the story: the future is already written and that preordained future is irreversible. If you've read other reviews, you probably know that it works in reverse-chronological order; this structure adds weight to the philosophy by showing you the future first, which makes for an intriguing experience, but it causes the film to haunt you even more without the typical closure delivered at the end of a revenge movie.
For me, this concerned a difficult subject matter that I am extremely sensitive to. View at your own discretion.