If you are still in doubt if you should buy Mario Galaxy, know that you are losing a really great game!
Super Mario Galaxy has a lot in common with Super Mario 64, with all that stuff about collecting stars and complete some tasks in the many worlds (galaxies) of the game. The difference is that the game goes on in a whole sideral space thematic, with greatly designed levels,each one with their "own gravities" what can make Mario be walking upside down in spheric planetoids or in other kinds of giant floating forms (believe, this is more awesome than you think). All of this can make possible many kinds of puzzles and challenging situations.
Talking about "challenging", in fact, Mario Galaxy isn't really a hard game. If you are a veteran player, you won't be finding any difficulty in get the necessary number of power stars to finish the story (but there is many really hard tasks to do after). Nintendo did this on purpose to attract the more casual public. You can note this looking to the simple controls and the null learning curve (Probably even your parents will like playing this game). But all this stuff had a price. Mario Galaxy doesn't have all those interesting secrets like Mario 64 ones. The objectives are mostly linear and easy to get to and there isn't any secret worlds, except the Trial Galaxies (IF you can call those "secrets").
But, forgeting all these problems, Super Mario Galaxy is great to play, has one of the most beautiful graphics of the Wii (this is the only Wii game with graphics comparable to the X360 ones) and has an excellent orchestrated soundtrack. And if you want to try, there is a, at least strange, Co-op multiplayer mode, where the player 2 only controls a second cursor on the screen, collecting star bits and helping Mario to jump higher.
Concluding, SMG is a must buy, a really great game, but a waste of potential in certain points. I hope they fix these flaws in Mario Galaxy 2 and make a game able to match, or even to overcome the classic Super Mario 64.