With only limited time to play the game, I've finally managed to unlock the ability to play as your Mii within Mario Kart. My son just absolutely loved it, and I get a big kick out of it as well. It essentially gives you the ability to play with an unlimited amount of characters within the game. It's especially fun online where you have people that may pick several characters at the same time, so you can avoid having four Yoshis or something on the same map.
Mario Kart continues to be a lot of fun, and should be even more fun as my friend list grows. Just got codes from Mike and Laura, as well as Greg, so we should get some games with friends going soon!