I had seen bits of it before, but I just finished watching it.
It's not an extremely known movie (well, I don't THINK it is). It's a suspense/murder. Not really a horror (it's creepy and sometimes violent- but in a "sophisticated" way- and disturbing. NOT outright exorcist-frightening). I was amazed by its storyline and found it a peak of cinema. Discuss it. If you haven't seen it, I suggest you do so (warning: about 12 seconds of sexual content, swearing, disturbing images, and violence).
It's similar to Hannible or Fracture if you've seen those movies. Here's a basic summary of the storyline with SOME (but not many) spoilers.
Spoiler:
Mr. Brooks is a successful owner of a box-manufacturing company. Voted "man-of-the-year", he has a wife and one daughter. However, his split personality influences this genius' every move, and has made him a homicidal serial killer. It's been two years since his last murder, and now Mr. Brooks is having difficulty fighting off his "friend."