One of the best platformer games ever.
Most of the levels are really fun and makes me play them all more than once, even when I've already fully completed them. There's only a select few levels in the game that I didn't really like. Two of which involve a bird named Fluzzard, who you just fly with through the whole level. Not that fun to me. The game has 242 stars to collect altogether, meaning there's a lot to do. There's even green stars to collect at a certain point. These can be hidden anywhere, even in some obscure places. Adding these was a pretty nice idea. Another cool thing is that there's even remake of a few levels from previous games, including Super Mario 64 and the first Super Mario Galaxy. Although, I wish I would have seen a few 3D remakes of levels from the old 2D Mario games too. The bosses are fun to fight too, notably Bowser himself, Rollodillo, Megahammer and Squizzard.
After beating Bowser for the third and final time, you unlock an additional world, which can take a long time to beat, simply because of the very last level. It is one of the hardest levels ever in a Mario game and by far the hardest in this game. It took me about a couple months until I was able to beat it. They really saved the hardest for last here. They really did go a bit overboard here though. But since the level isn't even part of the main game, I guess I can't complain much.
There's also new power-ups, such as the cloud, which allows you to reach greater heights by summoning a cloud to stand on, and the rock, which allows you to form into a rock to break certain structures that couldn't otherwise be broken. One great thing is that Yoshi, whom was missing in the first Super Mario Galaxy, returns and is kind of like a power-up himself and even has a few power-ups of his own. These power-ups really adds fun to the game. They should have let you bring Yoshi to any level you want though, which you could do back in Super Mario World.
I like how you can now navigate the galaxies through a map hub world, much like the ones seen in Super Mario Bros. 3 or Super Mario World, instead of the hub world seen in the first Super Mario Galaxy. The hub world itself offers things like gambling for more lives or starbits and shifting between Mario and Luigi once you have fully unlocked Luigi. I say "fully", since some levels will ask you if you want to play as Luigi beforehand, which is a nice and interesting feature.
This would have to be my favorite game of 2010 and my favorite Mario game ever. I rate the game a 10/10.