The ending (spoilers)
This movie was pretty good, but the ending bugs me.
It becomes pretty obvious that they are delusional at some point in the movie. To me this moment was around when Agnes told her friend to never come back. After this point, I feel the movie drags out unnecessarily. The part when they have tinfoiled the room is way longer than it has to be. I feel we already know by then they are delusional, and there is no need to prove it by showing Agnes and Peter going more and more insane.
Also, I don't like how they made Peter stab the doctor. At that point I knew the doctor was the good guy, and I hoped the main characters would come out of their delusion. Peter killing the doctor doesn't seem in character, there seems to be no trigger to change his behavior so drastically.