A really good, scary film, in the vein of The Blair Witch Project, but in a house. Personally, I reckon the performances were spot on; the characters and their reactions made sense to me. They didn't necessarily do what I would have done, and I have a few gripes, but that doesn't affect my recommendation that you go to see the film if you want to see a good horror/thriller.
I think you'd get more out of it if you know a little about demons, hauntings, etc.
(ryan, of course it's a fictional. They're not going to show a film with real demons. If you want to see ghosts, watch Taps or Ghost Hunters).