Excellent film. Don't listen to the others they are people with short attention spans and are used to the schlocky cheap horror that is all too common these days.
The pacing is a slow crawl, which will obviously turn weaker minded people off, but every detail is important and the films direction keeps you engaged with some memorable imagery to boot. The films score is as menacing as the subject material, I was shaking with anticipation the entire way through.
I don't want to give much more detail without spoiling the movie, but this is simply the kind of horror film that just doesn't get made these days.