The central character is labouring under a fundamental delusion, one that will change the way the viewer sees both the character and the plot, and in fairness, this delusion is resolved gradually and masterfully. Despite this, even before I had worked out for sure what was really going on, I considered getting an early night instead. I wish I had, and here's why.
Despite excellent acting and direction, I found the movie unsatisfying. The viewer is fed the central character's same delusion as "reality" from the outset, with a view to being let in on the truth as the movie progresses. This leaves me, personally, with a feeling that I've been taken for a ride. Not the worst ride I've ever been taken on, but one that reminds me of Stephen King's assertion that an author should not intrude on the plot. I feel that in creating a false reality for the viewer, the writer intrudes, cheating us by laying out red herrings to maintain interest and mystery. In doing this, they conned me out of my early night.