Buy it.
If you don't have a Wii, get one of those first. Then buy this game. It's just that good.
Nintendo seems to be moving towards games that encourage interaction between people of different skill levels playing together, and SMG is no exception. Imagine Super Mario 64. Now imagine that each room in the castle is a galaxy of planets to explore. Now imagine that, instead of playing lives and levels, your gaming companion can pick up an extra controller and pick up bonuses, stun enemies, and -- if you get the timing right -- help you do a "special co-star super jump." That's basically Super Mario Galaxies.
It's truer to SM64 than some of the more recent games, like the Paper Mario series or Super Mario Sunshine. It's a straightforward search-destroy-collect game, with the odd minigame (think race and puzzle special levels) thrown in for good measure. There are some puzzles to solve, some bosses to defeat, and worlds to unlock. Nothing groundbreaking. But it's fun. And it makes really good use of the Wii controller (P1 needs the nunchuck but P2 doesn't), especially for P2.
It's possible to play alone, but it's more difficult and less fun. I'd be happy to play P2 the whole way, but Pedro insists we swap out on level changes so I've had a fair amount of gameplay on both sides. If you've never played a Mario game, this would be difficult, but still fun. (If you've never played SM64, you can download it from the Wii Console for about $10. Well worth it.) And if you're an old-school Mario fan with a SO who gets annoyed that you're always playing video games without him/her, this is the perfect solution. P2 requires very little skill but a high level of involvement.
Much fun for two. Four thumbs up.