It's a very well crafted piece of cinema that intelligently deals with the idea of detectives chasing down a psychopath. The characters are well defined and well acted, and the plot isn't really 'guessable'. It's Brad Pitt's best movie.

As others have said though, it is definitely a bit gratuitously sickening in places but not in a way that's out of step with the plot of the film. It almost crosses into the horror genre in places (in a Hostel or Saw franchise sort of way). If you're squeamish it's probably best to stay away.