Mario Kart 7 is an okay game.
There isn't much to do in single player (I don't even count vs. mode since it's a snorefest), so you'll be spending most of your time online (when you're not grinding out 5000 coins, 40 at a time, for new kart parts).
Some feel the Maka Wuhu glitches ruin online, and it kinda does. It's basically do it or lose, and it's a shame that it's come to that. I've also heard that it's pretty difficult to stay online for any period of time, and in my very limited experience with online, I can say that that statement has some merit.
If you don't have wi-fi capabilities, I would skip this game and buy Mario Kart DS instead. It has a much more solid single player mode (and R.O.B. and Waluigi, for what it's worth).