After watching a live gameplay stream for the past 4 hours, I have to say... this game is epic. The level design is fair, the bosses can be challenging (but once you figure out how to beat them, it's not as hard), the puzzle elements are exquisite... The world map is a nice touch. Also, the music fits each level perfectly. Whether it's slow and calm for a water/ice galaxy, or fast-paced and awesome for a HURRY UP AND MOVE YOUR ASS galaxy, it's great. The reorchestrations of classic tunes is well-done too. Many things are recycled from Mario World and Mario 64 as well. And instead of waiting around for comets, you collect Comet Tokens (already forgot what they're called) to trigger their appearance. The humor is intact as well.
One last thing. The first Bowser fight? Epic. I highly recommend this game.