It's not really about exorcism, though it is about possession. And I have to admit, that I am surprised by the low rating here on IMDb. It's very well crafted, has an array of good actors in it and the script is solid. While I would agree that it doesn't reach the heights of some of the best horror movies out there, this is certainly a very good movie.
The acting is strong, the story holds up and the ending works too. Yes some characters react strange to our main character, but overall it's not the worst behavior in horror movies (or in the clichés we are used to). I like the mentality of the movie and the director seems to know his stuff. As does the cinematographer (both plus leading actress are on an interesting audio commentary on the disc) too. I was pleasantly surprised by this and maybe you will be too