Memento is a movie that deals with the human memory and its reliability and encounters this topic with an unique structure of narration, that perfectly fits.
I don't want to spoil too much, but the creators of this masterpiece came up with a way of giving the audience an experience, that comes close to what the protagonist is experiencing thoughout the story.
It's thrilling, fascinatiing, surprising and just a one of a kind piece of genius. If you like intelligently made movies with a few twist and turns you have to go and watch memento.