Saw the twist coming miles away...
Mostly because this has been done numerous times before. Fight club for example.
There is another movie that comes to mind, however I can not remember its namn. Its about a teenage girl (and her sister, who is a figment of her imagination) who suspects that their "fake" mother is "evil", however it is the teenage girl that is mentaly sick.
Anyway, shame that I saw it coming, I assume the movie would have been better if I hadnt.