Excellent game. I'm more into GTA and FPS kind of games usually, but I have to agree this is outstanding. The quality and sheer quantity of things to do is dazzling. This is a huge game in all meanings of the word. It can keep you playing for weeks (or months for completionists).
Also, one thing often overlooked in the Super Mario Galaxy reviews is that the game is graced by a "just right" (for me) difficulty level. Actually this means "pretty easy" as I'm no fan of having to restart levels from scratch and very difficult bosses.
But here, the game very intelligently throws extra lives at you in a pleasant way and put bosses that, even when they present some challenge, are rarely that difficult and never too long. This is a big plus for me and one of the reasons I loved the game. (it's also a reason I usually dislike japanese games, often unnecessarily difficult). This one played like a breeze ! A milestone in gaming.
My only "negative" is a lack of immersivity, with a world and levels that are too "abstract" to my liking, making it difficult for me to get a sense of being in a real place. But I accept this is part of the Mario setting and style, a fantasy world for both children and adults.
9/10