First off, I would say that I am a huge horror fan. I try to appreciate each movie on its merits (whether it be terror, suspense, or, even, camp). However, this movie attempts to build up the suspense through its "Blair Witch"-style filming, but it never reaches the apex. Most of the movie focuses on the personal choices of the Evangelical pastor, but, by the end of the movie, the viewer still doesn't know why he/she needed to know so much about this character. The entire movie moves very slowly except for the fleeting scenes of possession (each not lasting nearly as long as it should have.) As other reviewers have said, the best parts of the film were all used in the previews. The end of the movie involved a mixture of "The Omen" and "Rosemary's Baby"---but without following through on the scare factor. Ultimately, I would recommend that you pass on this movie.
If you're looking for excellent acting, this isn't the movie for you. However, if you like something action-packed and very "jumpy" horror, watch this movie.