I felt that the main character (the young priest) would have benefited from a stronger actor. I also think that Lucas's possession should have had more lead-up and tie-in with his character, why he was possessed, etc.
I didn't think it was trashy or tacky, but I think less effects on Anthony Hopkins and more dialogue / content to play with, because he really doesn't need the special effects, he's a good enough actor himself to pull off what was needed to make this a potentially great film.
The build-up before Lucas's possession is good. Anthony Hopkins is the driving force in this film. He does well with what he's given, and a lot of films have done worse in not serving up enough content (I don't mean pea soup, I mean emotional input), this film does a reasonable job.