I looked this movie up on "Rotten Tomatoes" movie review website. It got like a 40% positive rating from the critics, and about 95% positive review from viewers. I wonder what the critics didn't like? I'll need to look at their comments, but sometimes the critics are way off. I watched the trailer for the movie, and it looked pretty intense.
It has God v Satan overtones running through the movie and even points out in one place that Satan is winning the battle. Most of the souls here on earth belong to Satan by default, because they don't belong to the Carpenter. I doubt critics like that kind of talk.
It's not a feel good movie. At one point a priest is brought in, who doesn't believe in demons, and the prisoner just laughs at him, stating that if you don't believe in the devil there is no way you can claim to understand God. The psychiatrist claims to be an atheist, and claims that since he is an atheist, he is immune to such beliefs about God or Satan.
The demon just totally starts fucking with him at that point, and by the end of the movie has taken possession of him too.
It is like the Exorcist. Except the Devil wins in this one.