Just wanted to update for anyone sitting on the fence (and to tempt anyone who has the game under the tree and not in their Wii):
After you open up your fourth observatory, the game just becomes AMAZING. I mean, the first 20-30 stars are fun, but starting with the
the levels become long, complex, and require both problem solving skills and good to great reflexes. I loved Mario 64, but SMG is a whole 'nutha level -- this is easily the greatest 3D platformer ever made, if for no other reason it feels more like a 2D platformer with 3D elements rather than the usual exploration and collect-a-thon that almost all 3D platformers are nowadays. The level design is absolute genius -- I am constantly muttering, "that is *bleeping* amazing" all the time when I play because the cleverness just gushes off of the screen. The ingenuity is endless, and there are some levels that have 3 or 4 moments where your admiration almost distracts you from the task at hand.
SMG is not meant to simply be beaten. You owe it to yourself to at least try every level, because it's a despicable waste to get just enough stars to beat the game and then leave all that untapped genius untouched, especially when the better levels open up after you have the minimum amount needed to reach the ending. Don't worry that this game is too short. It will only be short if you cheat yourself out of the privilege of trying every single level.
SMG is one of those games where the hype is totally justified. And it should be believed.