The overall theme of the movie has been done before, which the movie itself even makes a comment about. But that is not the reason for 2 stars.
This movie is just hard to watch. I love horror and do not consider myself particularly sensitive to gore. But the elements of slow torture made me almost turn the movie off. If you want the same shock factor and amazing twist, skip this movie and go watch Cabin in the Woods. They did it first, and better.