I played the demo at a Gamestop here in Chicago today (the Gamestop on Fullerton by the Chuckie Cheese, if anyone cares).  I found the game to be very fun.  There were 2 selectable worlds, some "Egg" galaxy, and the "Bee" area.  I played the Egg galaxy.  I'll keep my impressions short.  I played a round of Mario Sunshine last night, and I found Galaxy's controls to be dumbed down a bit.  There was no quick-jerk side jump backflip move, which I apparently  used a lot, because I was constantly trying to do it in Galaxy.  Otherwise it feels great.
-Mario seemed a little slower than in Sunshine.  
-Blasting from planet to planet is a serious joy.  
-The TV at Gamestop made the game look terrible and pixely (I can't wait to play it on my low-def set at home).
-Secondary Wii-remote functions (gathering star bits and shooting them) were totally intuitive and fun.
-The game is action-packed.  Finish one task, move on to the next.  Really fun.