This is quite possibly the best game Nintendo has ever created.
The Bad: Some of the power-ups and genius design choices aren't used as much as they could be.
Super Mario Galaxy is an amalgamation of everything right with Nintendo and all its shady Wii schemes. It looks absolutely beautiful, it knows the limitations of the control scheme and uses it to its advantage, and it provides comfort that, indeed, Nintendo knows what it's doing. If the entire Wii generation ends up comparable to SMG, we're in good hands.
While playing Galaxy, it is impossible not to realize that a lifetime of talent and practiced polish have led to the creation of perhaps the tightest gaming package ever. The game is an absolute marvel to play for Mario newcomers and veterans alike. It is chock-full of interesting new ideas and gameplay mechanics, and all of them work marvelously.
For example, the very first thing introduced to players is the new gravity mechanic. Often, Mario will find himself on a round planetoid, and will be expected to navigate around it. The ability to move around and not fall off anything gets rid of that age-old fear of accidentally dying through a navigation mishap.
In fact, the greatest strength of the game is that it fixes all of the problems with the previous games in the series. The camera is a work of art, the difficulty is perfectly suited to all players, there are loads of objectives at any given time, few (if any) of the stars are dull to obtain, and there is so much variety that you'll never get bored playing it. The amount of levels is staggering, and you'll never find yourself doing some of the mundane activities found in Super Mario Sunshine.
The only problem with Super Mario Galaxy (a fixable one, at that), is that you're going to see things in there that you wish were expanded upon. At one point, you will gain a power-up that allows you to fly. It is very fun and intuitive, but it is only used for one star. If Nintendo offered a download pack at some point with some new levels that centered around this great power-up, it would be solved.
The point still remains that Super Mario Galaxy blows all of its predecessors out of the water and sets expectations for coming Wii games sky high. It is a truly out-of-this-world experience that deserves to be listed among Nintendo's best games, including Ocarina of Time. Go play it now, if you haven't already.