Psychonauts offers one of the greatest experiences to be sure; it's just not always the greatest game.
Where does it go wrong though? In the laziness of the gameplay. Everything about Psychonauts seems sharpened like a razor except the gameplay. Too many unresponsive moments. "I grabbed it!" "I jumped!" as the age old adages of platformers go. There are many many many of those moments to be had here. Your face might go red at how unresponsive it is at times.
That's my only gripe though. The character-driven story is hilarious. There's rarely ever a forced gag, and most of the jokes/shenanigans are flat out laugh out loud. This might be the #1 reason to have this game.
Second, the design is genius. You think you've seen it all, but each level seems to be more imaginative than the last. There's never a moment where something seems uninspired or trite. Not to mention, all the levels are good long treks.
The leveling up system is also quite fun. Basically, it's a collect-a-thon, and depending on the more things you collect, the more psychic powers you can get through leveling up.
If you really want a good description, I guess I might go with Zelda, with a larger focus on platforming than anything.
So, if you love humor in your games that is actually fresh and funny, Psychonauts is probably the best game to get. It's level, character and world design are also brilliant and will keep you playing just to see what the next mind looks like. However, it suffers from some major unresponsiveness that might even have you screaming at times. But I still recommend it.