Fight Club is so gruesomely, graphically violent that we wouldn't be surprised if the Blu-Ray itself jumped spinning rapidly out of the case and cut your head off. Be careful.
Men get their faces beat in, teeth knocked out, eyes blackened, and noses broken. People are shot in the head (twice or maybe three times, it's hard to tell when two people are technically the same person) and multiple people almost get castrated.
There's also some frank sex talk, a computer-generated sex scene, and some not-so-subliminal images of some decidedly not castrated body parts. Fight Club pushes the limits of the R-rating in a way that only a movie pre-Janet Jackson's boob-slip could.