Darren Aronofsky, the director, uses the same unsettling camerawork and editing style he later used in (the superior) Requiem for A Dream, with the aid of superb dialog and acting, notably Sean Gullette in a lead role and Mark Margolis who plays the friend, and gives my most favorite of his performances.
There are some unexplained things (e.g. the bleeding man, the brain), unless it is a way of showing the mental suffering the lead character is experiencing (update: an explanation was given). The movie starts a lot better than it ends, and although made by a stylistic master-director, there was at least two sleep-inducing moments.