I'm a little ways in. The game is fun, funny, compelling, visually stunning, and very much a success! Hooray!
What's surprised me is how much in-step it is with the first game, like explicitly so. Not only does the story pick up right where Rhombus left off (which in turn picked up directly from the end of 1), but you start the game with the powers and abilities you acquired in the first. Play feel is the same, the collectables are the same, the level design philosophy is the same. It's the same game experience but a little tighter, a little more polished, and with the benefit of 15 years of technological advances.
What's interesting is this extends past mechanical design. Premise and plot is directly echoing the first game. Despite 1 ending with Raz recognized as a full Psychonaut, here he's once again positioned as an outsider at the bottom of the social order and has to prove his worth to the rest of the cast. Once again there is a group of students with individual personalities that make up the supporting cast. One again a mysterious antagonist is stealing brains. All these repetition feels to a point and I'm curious if the game is using familiarity with the first to set up big deviations later.
However, if it a repetition of the first because Double Fine understands sequels as the same game-experience "but more!" that's okay because, one: it's a great experience, and two: I think choosing to retain so much of the design structure clarifies how the first game was an Adventure game told through using the language of platformers, rather than the other way around.
Regardless of all this design talk, the game has a lot of value purely as a spectacle. I've only done a couple mindscapes and already there have been some real sights to be seen. I can't wait to see what they have to show us next.