The Exorcism of Emily Rose was one of those pleasant surprises. You know, you rent a movie and you think, "well, this will probably suck, but I'll give it a chance...."
It's increasingly difficult to know ahead of time what to expect from a modern horror movie. You can't go by the "critical acclaim" quotes on the DVD package.
Probably one of the best places to look is Rotten Tomatoes, but even then you may find a movie you really like that most critics panned--like this one, which only has a 44% rating.
Anyway, this is a good movie. Surprisingly good and very creepy. Don't watch it with a bunch of your friends on a sunny afternoon. Watch it in the dark and turn off the phone and the lights.