I really liked this movie. Many of my favourite horrors are ones that lull you into thinking they're not even a horror movie for the first act or so. In fact one of the reasons I seem to enjoy horror more and more is that they often do a better job at storytelling and characterization than most other dramas these days.
I was really fascinated by Michael's background and his internal conflict on choosing his career path, especially his doubt and his skepticism toward exorcisms and his faith in general.
Once things start getting creepy, you're really invested in it. As Michael's skepticism gives way to fear, it seems entirely believable and the creepy factor really sinks into your bones.
SEMI-SPOLIER COMMENTS FOLLOW...
The plot took some unexpected turns that I really enjoyed - the subject of the exorcism turned out not to be the main plot (where is the movie going from here?? I was wondering.)
The main downside to this movie was when the attention turned to Anthony Hopkins character. He turned it up to full-blown posession acting, and it sort of snapped the immersion that had me going up to that point. After that it's a lot of shouting, slamming doors etc. etc.
The first 2/3 of the film was great IMO, I just wish they had finished it off a little more ambiguously, with the nuance and intelligence that the rest of the movie had.