Just some quick stuff until my review gets written, edited, and published:
24 total tracks, several, probably five or six four course grand-prix modes on beginner, at least. 64 single-player missions, which consist of driving to stuff in one lap around a track, or shooting specific targets, or just a two-lap race with a fixed character. The definite "best" challenges have you steer or control a racer in their "All-Star" attack mode, which is about on par to Brawl's Smash Ball.
Past that, unlocking takes a "while," I suppose. I racked up about three hours of actual play-time and still have five characters left, though I saved the quickest five for last. Stages and musical tracks aren't yet available in the store, so I haven't unlocked any of those yet. I believe only the first four stages begin unlocked, the other twenty, you likely have to buy. There seems to be 40 possible tracks, if an "achievement" like meter is correct.
Beyond that, online mode is interesting. Inside an individual race, I don't think it could be any better. I haven't even noticed any "jumping" due to lag in position. I've raced with probably four people though the number of people in the lobby before the race, and the actual number who participate often wasn't the same. The only problem is that the rules the host suggests are final. You can't choose to host the game, nor can you pick a specific game to join, so if you want to race without the AI players, or with them set to a specific difficulty, or even pick the track to race on, it's a real crapshoot, until you "receive" the ability to host. From there, as long as you don't quit or disconnect, you retain control. It's a bit weird, and annoying, really.
As to better or worse than Mario Kart? I'm not sure. It's new, but very derivative work. The real unique Mario Kart stages are typically better than the more unique stages here, but the average SASASR track might just be better than the average Mario Kart stage, except for the loops, which are just stupid.
Beyond that, I'd have to say... hmm... I think what everyone knew: Pick it up if you like Mario Kart and want more.
Oh, and the framerate chugs along at most points, it feels a little jumpy, but the game's speed is constant and faster than Mario Kart. After some time, you don't notice it while you're playing. I don't know how that holds over for the PS3 or 360 versions, but I've read they might be the same way.
Edit: Whoops, you can press the "1" button in the shop to switch the category you're buying, making stages easier to buy than I stated =P