The found footage horror film is nothing new. The Blair Witch Project and Paranormal Activities have both succeeded with this formula, while others have failed with this now overdone trick. The Last Exorcism is another in a long line of these doc**entary style movies, with one exception: it's actually pretty good, and damn scary.
Cotton Marcus is a preacher about to perform his last exorcism. He brings along a doc**entary crew, in part to prove how fake these things actually are. He heads down to a remote farm in Louisiana where the livestock has been dying off, and the patriarch of the family thinks it is his daughter, Nell, who is doing this. Of course, with a setup like this, you know where it's going to head, and it does, with Nell contorting into all types of ungodly shapes and Cotton questioning what he believes.
The gimmick works in this film for one reason: acting. Patrick Fabian plays the reverend with a lot more conviction than you would expect from someone in a horror film. His belief that what he has been doing all along is hooey runs headlong into the new reality he's facing. Fabian brings that across, so when the film falls into familiar territory, we still go along, because we CARE about this man. There's also enough genuine scares in this movie (and, surprise, they're supposedly not of the CGI variety) that even when the end leaves something to be desired, I didn't feel like I'd wasted my time.