The best Wi-Fi experience on the Wii. The motion controls are good too.
Now let me be the first to say, I love the wiil. At first it sounded kinda dumb to me, a remote in a plastic shell, but it's actually a lot of fun, and quite responsive too. I never went back to my GC controller, if that says anything. I'm not a big fan of gimmicks, but the Wii seems to really do these well, and I'm more about having fun than being the best.
But this game is also geared well towards the pro-audience as well, mostly with it's tournaments and ranking system. These tournaments usually come 2 a month (sometimes less I think), which vary in how fast you can complete a challenge. These challenges get creative, so it's a little more than just your average tournament bracket stuff. There is also a Mario Kart channel to tell you when a tournament is underway.
Single player hasn't changed a whole lot, but is still fun to go through once or twice, especially to unlock the stuff for online play. The battle mode has changed somewhat, aimed for more people, but is still fun nonetheless.
Wifi mode mode works really well. Your main objective if to gather points in races... and you can race with your Mii if you wish. The higher you go, the harder it is to earn points. I'm in the upper 7000s. Personally I like using nintendo characters, but the option is there. Leader boards are also present if your into ghost racing, and is well suited for the hardcore crowd. I also hardly notice any lag. I can connect 90% of the time without issues or waiting more than 30 seconds.
The gameplay, again, is very similar to previous karts. You race around a track, get items to blow the other people up, and thats it. The game has 32 tracks, 16 of them being throwbacks. I always preferred the N64 tracks when it comes to the old ones, but the new ones are outstanding on their own. Tricks are also new to the series, giving you a boost off a ramp. And bikes are a good option for skilled players. A minor complaint I have can be the chaos of having 12 people per race. It is fun to see more items flying around at once, but can be frustrating too. And the items sometimes feel a bit too powerful. Then again, I think the items have always been that way. Its just now that you're playing real people, and not AI, staying ahead is much tougher. I DO prefer this over Mario Kart DS though, which only allowed 4 players, and half the time you didn't even get 4. That was slightly boring.
In the end, I think you're getting a good deal here. The game is just plain fun, and tries it's best to please everybody. This is what I expected from a Mario Kart title, and maybe even more.