I'm loving this game. I logged almost nine hours (yes, nine) yesterday. Played a bunch during the day and then most of the evening with friends.
I thought I'd put the wheel down after a short time, but I'm surprised that it works really well. Most games, the motion controll seems to go wonky from time to time, but this works perfectly 100% of the time so far.
The online component is amazing and so much better than Smash Bros. It adds an incredible amount of playability to the game.
I finished the 100cc with little problems, but 150cc is pretty annoying. But, unlocking the Mii drivers was all I really wanted, so I'm happy.
Most of the new courses are really good. I do think Rainbow Road was designed by a sadist, who wanted me to fall off every 10 seconds or so, but I'm getting used to it. Wario's Mine is tough too.
Most of the old tracks are okay. Can't fathom why a few made it in, like Ghost Valley. It's short, it's boring, it's worthless. Overall, it's great to have some retro tracks in there. Too bad the Wii has such little storage space, as otherwise I could see Nintendo releasing more classic tracks as downloadable content.
I'll post my Wii and Kart friend codes later, and will add anyone else who posts theirs.