I saw The Last Exorcism at the LA Film Fest tonight. Loved it. It's a fake documentary in the vein of Paranormal Activity, which I loathed. Exorcism justifies the constant presence of the camera, pulls it off seamlessly, and uses it to great effect (as when they all go to sleep and turn the camera off, and then it comes back and we realize we're in the POV of the possessed girl, who has risen from her bed and stolen the camera, which she uses to bludgeon a cat to death while still filming). It's well-scripted, creepy, very much a Louisiana film, and scary as shit.
The screenplay is the foundation--light years better than Blair Witch or (certainly) Paranormal Activity. There's a really excellent and ingeniously constructed second act in which, without giving too much away, the father character's motivations are perfectly set up (and a great sequence, the best in the film, in which it seems he's about to kill his daughter, and we see him from inside the house crouching and praying outside, as the frantic documentary crew tries to decide whether to flee immediately or risk death via shotgun by trying to find, subdue, and rescue the maddened daughter before the father can kill her). The script does a great job of parsing and dispensing crucial information in an OH SHIT way that doesn't feel contrived. The very end--maybe a little much. But I didn't see it coming, and it is ingeniously set up, so I admired the writers' guts.
The trailer, which makes it look a lot less nuanced than it actually is. (The preacher actually doesn't believe in the literal power of exorcism, and he's performing one in order to demonstrate the power of suggestion and reveal the smoke and mirrors that exorcists use.)