I'd say the particular levels themselves were fairly linear but there is alot of collecting to do in the open areas. The collecting only becomes tedious if you seek out all of the figments, but the Scavenger Hunt Items and Psi-Cards were always a pleasure to seek out.
I can't believe the anti-Psychonauts sentiment on this board. I understand the game did not sell well, but I think that the game was extremely original and a blast to play. I think nemo's point to get it on the Wii is a valid one. People may get to appreciate the excellence that was Psychonauts if Nintendo really pushed the game(perhaps published it). Fanboys like us eat up pretty much anything Nintendo formally endorses and I don't see why a Psychonauts 2 would be any different. Although the likelihood of any of this is slim to none, it's still nice to dream.