This isn't a masterpiece, and this will not blow you away like The Departed. That being said, this is still clearly the work of someone who knew what he was doing. The movie keeps a continuous aura of "What the hell is going on?" going with surprising finesse, and eventually poses twists and a conclusion that are not only satisfying but redefining. In other words, this is the sort of movie that you could watch a second time and see tons of things you did not not notice before you'd seen the ending. Story-wise, this is a mind bender that ranks among the best.
DiCaprio, while not as good as in The Departed, displays his full talents here. The movie would not have been as good or as powerful with a less capable actor, that is for sure. The cinematography is beautiful, with many memorable shots and artistic composition. The editing is sometimes a bit strange, as Scorsese cuts back and forth between shots of characters in different positions, which is either because of the whole insanity theme, or because he wanted the very best performances. At any rate, this is top-notch technical work.
All in all, while not the best movie we'll see this year, it's nonetheless a really excellent and really crazy movie that will mess with your mind and won't let go even after the credits roll.
In a sentence:
An incredibly mind-bending thriller held up by top-notch cinematography and acting.