Best. Cart. Racer.
MNR converted me from Mario Kart, and now I don't think I can go back to Mario ever again. This game was just so satisfying. Anecdote: I bought the DLC for Mod Nation Racers, and I never, ever, buy DLC.
The racing is fast and energetic, but still has that "carty" feeling. The controls are wonderfully tight and that's a good thing, because you're going to be drifting like a madman. Beating all of the tracks and challenges in sinlge player is challenging but fair, and satisfying. I had a blast in single player.
Online play is great as well. I sometimes steer away from competitive play, but I couldn't put this down. The game has a leveling system, and you are awarded experience points from playing online. The better you perform at placing, fighting, and tricking, the more points you get.
The combat aspect remarkably similar to(clone-like) Mario-Kart, but probably better balanced. No blue shell needed to keep a race artificially "close". Still, even without blue shells and bullets, you don't feel out of the race being in 6th place in the last lap- just don't count on magic items taking you from 12th to 1st place in the second half of the last lap.
I probably won't say much about the create and share aspect. I only tinkered with it, but the creation aspect was robust to say the least. There are an intimdating number of unlockables for creating tracks, cars, and racers. It didn't make or break the game for me. Some people seem to love it.
That is all.