Saw the Last Exorcism this weekend and give it a strong recommendation. Its not your typical exorcism movie, instead it follows a pastor who doesn't believe people are actually possessed, he thinks its a symptom of small minds from small towns. He uses some slight of hand to make it look like demons are talking, their kids feet are making water boil etc etc, then does a fake exorcism.
He lets a film crew follow him on one such event, but things get complicated fast. One of the nice things is you can never fully tell whats going on. My wife seems to guess the ending to even the most complicated/twisting movies but she was guessing till the end of the girl was possessed, abused, mental or what.