One of the more enjoyable "platformers" around.
One of the best aspects of the game is the completely different galaxies that you must progress through. Every one is different in some unique and interesting ways. Some allow Mario to find and use some interesting power-ups: fire, ice, spring, and bee. So the game is more than just simply running around avoiding baddies.
SMG makes good use of the Wii remote, allowing Mario to spin to accomplish various maneuvers.
My only complaint (if I had to give one), is that the galaxies reachable from the engine room (the last set of galaxies you can plan) are too difficult. Some require replay after replay until you memorize the level -- something that is thankfully absent from the rest of the game. That said, I didn't actually need to get through these galaxies to finish the game. I had found some secret stars earlier and so I beat Bowser before needing to complete these galaxies.
All in all, a super enjoyable game. I highly recommend it to everyone.