I went and saw it last night. It's definitely more a thriller than a horror movie. One thing people keep thinking as they go in to see it is that it's a movie about an evil kid. By the end of the movie, you will realize it's not that kind of movie at all.
I absolutely loved the makeup job on the 'kid' at the end of the movie. They really managed to do it very convincingly. You'll see, when you see the movie. The acting is pretty good. The characters were perfect for the movie. The asshole, former cheater husband, the mother who has had some psychological issues, the suspicious older son, and the adorable little daughter. And, of course, the psycho adoptee.
It was well shot and well written. Some things seemed to go off on a tangent, and while nothing was left unexplained, occasionally the explanation was a little weak.
I think the worst part of the movie was the theater experience. The theater was FRIGID. The seat had this wierd, damp sensation, and three teenage girls who sat in front of me giggled and whispered the whole time, and of course shrieked and giggled at what they imagined were jump scenes. It was rather annoying.