We've seen psychological thrillers with a twist, but Shutter Island expands the genre one step further. In The Sixth Sense, A Beautiful Mind, or The Others, a crucial story twist suddenly gives every plot element a different meaning. Shutter plays a very different (and mind-boggling) game: At the end, is DiCaprio really ___ , or is he not? Scorsese makes us as the viewer doubt our own sanity. I always thought Cape Fear (another psychological thriller) was Scorsese's best. Shutter proves Scorsese's as good at this genre as gangster movies. Very highly recommended, even though it'll mess up your mind.
(Warning: A reader advised me to warn parents about watching this movie. Not to warn about the kids watching it; to warn the parents about watching it.)