I'm nearing the end of Psychonauts, and its reputation is well deserved.
Really solid level design overall IMO and the platforming is good and at best forgiving.
It's understandable why some would refer to this game as a Platformer, but that is also unfortunate since it falls under the matnra of 'looks kiddie therefore platformer'.
Psychonauts is more of an Action Adventure game, akin to Legend of Zelda. There's mix of platforming, light puzzles, and abilities that are used for progression through the game. Thematically, the first half of the game really nails the feel of Raz training to become a Psychonaut with the 2nd half him going off doing proper Psychonauts missions.
So yeah if you hadn't played Psychonauts yet, do so if you like Action Adventure games. It starts slow but gets better as you get more abilities and progress through the game. I think the lengthy levels help pace the gameplay, as you can do 1 or 2 mindscapes a day, then come back to it the next to progress.