I've collected all 120 stars and had a blast with the game (won't spoil what happens after you get 'em all). I honestly liked Galaxy 1 better, because it had stages that felt more like whole worlds a la SM64, whereas Galaxy 2's stages feel more like Super Monkey Ball levels; just a bunch of platforms floating in a giant space defined by some sort of background, be it stars, sky, or what have you. Galaxy 2 does have some stages that just blow me away though. Aesthetically, Slimy Spring Galaxy may be my favorite stage in the history of the entire franchise. It's just brilliant, though I wish it had more playability apart from swimming with a shell. This game is polished well, but it isn't perfect. Again, my complaint is solely in the 'wholesome' feel of the stages, or lack thereof.
I understand their concept in reintroducing the oldschool "Worlds", each ending with a Bowser Castle and such, and seeing that applied to Super Mario Galaxy is wonderful. But I grew up on SM64, not the games prior, so naturally I prefer a large hub (like Peach's castle or Rosalina's ship) where you enter worlds from.
So again, incredible game, but not perfect by my standards. I honestly can't say I want to see another Galaxy game ever again after this one, and I doubt they'll make a third or anything similar. I prefer grounded play over spacey, gravity-based platforming. It was revolutionary in 2006, perfected in 2010, and hopefully absolved and absorbed by a succeeding mechanic in the future.