I am a HUGE horror movie buff, and this movie scared the crap out of me in parts! The beginning is slow, but I think it is somewhat crucial to give the movie its somewhat realistic feel. If it had started super scary, there would have been no place to go from there.
I have seen both endings, and the ending they have in the theatre is by far the best. While initially appearing overly dramatic compared to the rest of the film, it was fitting since it was the next natural step based on the previous events. My husband doesn't scare easily, and he was freaked out on the way home (now he's got something else to worry about - demons).
If you're looking for the (sadly) all-too-typical gore-fest that has come to pass for cinema in the past few years, this film is not for you. However, if you're looking for a movie that builds suspense, creates an electrical atmosphere in a darkened theatre, and takes your breath away (from fear), this film is right up your alley!!