Usually, with movies, half way through I can figure out what the twist will be at the end. I did not see this one coming. I'll just say this is more of a mental horror film than a physical one. (40% horror, 60% suspense/mystery).
For the people stating that they did not understand the twist or were wondering what actually happened at the end... you do have to really pay attention and look at the fine details, but if you do this, it will pay off and you will understand. I watched the movie 3 times, and realized something new each time.