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.