I think this game is fucking ingenious. Admittedly, the first time I played it I was on a fair amount of acid, but subsequent sober play-throughs have proven it still excellent. It's much less about the gameplay (which is... lackluster) nor is it about the humor (which I like most of, but can admittedly get a bit grating when you die a lot and have to hear the same jokes over and over) but rather it is about the conception of the levels. Even the tutorial levels, while not being amazing, are still well conceptualized, but once you get to the asylum the "mind games" kick into high-gear.
Trying to conceive of the multi-layered thought-process of just one of the levels boggles the mind, and to have so many well-implemented different kinds is beyond genius. I can't even describe how brilliant they are because laying them out in the linear fashion of the written word diminishes their impact. Jung, had he lived to see it, would probably have given up writing and just pointed to Psychonauts and said "That." It is less a game and more an interactive artistic portrayal of the inner workings of the human mind that happens to include combat and collectibles.
So, obviously, if you're into gaming for shooting stuff and high scores, you'll probably want to look elsewhere. But if you are willing to accept this thing on your console as a new means of storytelling, you owe it to yourself to play Psychonauts all the way through. And, if that is your paradigm, you should probably play Chibi-Robo, Stubbs The Zombie, and Killer 7 as well.