Psychonauts as a 3D platformer is pretty weak. A lot of people complained when you actually had to do platforming at the end (circus level), but that was probably my favorite level in the game. The level where you're plopped onto a strategy table is also neat.
Psychonauts has excellent art design and some levels are designed really well, mechanically the game is jank.
Psychonauts is a weak 3d platformer and yet the level design is still excellent post the intro stages. Milkman Conspiracy is one giant adventure game. It is excellent if you are a fan of that genre. The velvet bull level is fantastic as well, even as far as platforming goes, but especially in terms of story and level design. The Goggalor level is pretty much straight beat em up. Pretty fun. The theatre level is a pretty good puzzle to be cracked, and the Napoleon level is a great puzzle level as well.
Paychonauts' strengths are its storytelling, adventuring, puzzles, and secrets and powers, and level diversity. It would be true that Psychonauts is a weak 3d platformer, if it were actually a dedicated 3d platformer; it isn't. It's more of an adventure game where you happen to platform, Meat Circus aside. Psychonauts has only one bad level and it's that one.
Gosh, all of this highlights just how disappointing Brutal Legend was. It had none of the great storytelling of a Tim Schafer game, and none of the wit or charm. It was advertised as this action adventure game, turns out it's a boring rts.
I honestly haven't enjoyed any DoubleFine game aside from Psychonauts. Still haven't played Broken Age because of it.