As a Mario game: 5/5, but just not the BEST videogame. I really like it. It has many cool features, including:
POWERUPS: I really like the diversity of the Powerups, though I wish they had put more of them in each level. My personal favourite is the Bee Mushroom, wich makes you super light, gives you the power to climb on, yes, Honey, and even lets you fly!
GALAXIES: Instead of just plain old singular levels, this game groups them into Galaxies. Each Galaxy has 2 or 3 levels each, in wich your mission is to collect a golden Power Star.
LUMAS: A Luma is basicaly an alien shaped like a star. They are VERY cute, and they will give you often useful tips.
YOSHI!: For the first time in Super Mario Galaxy history, Yoshi, the green, hungry friend of Mario's, joins you on the hunt for Power Stars! Using his toung, you can: gobble up enemies, engage exclusive Yoshi Powerups, reach new hights, and more!
BOSSES: This game has some of the most fun and funNY bosses ever! One of my personal favourites is a GIANT snowball that rolls around his battle arena, where you have to hit him on the nose every time that he comes close to you; but my favourite boss is good ( well, tecnically, BAD) old bowser, who has some of the best
MUSIC: in the game. His final battle music is just: WOW. I. LOVE. IT. I can't POSSIBLY tell you what it's like, but I know for a fact that you'll love it to.
All in all, an AWESOME game.