Last night I tried to play a match through Nintendo WFC on Mario Kart Wii, and I was not disappointed.
Online, you can play with participants worldwide, within your region, or on your friends list. Because I don't have anyone's friend code yet, I just did a match in my region. When starting a match, you get a cute visual of who is joining the match, what their Mii looks like, and where approximately they are in the world. It seems to leverage the same look as the world in the Weather Channel. Additionally, those matched have a score close to yours -- you start with 5000 points, and win or lose points depending on your score at the end.
Once twelve participants are gathered -- and it looked like I was pulled into an existing match with 10 others, as I could spectate on another match as I waited to join -- the match initiates. You can chose your racer and kart, and then vote on a track. Then you're ready to go.
I also have to say this is very quick. It took less than 30 seconds from clicking the region menu item to the start of the race. Perfectly suited to fast play, and much more satisfying than the matching services on SSBB.
The race itself was great. Zero lag at all, even with two racers on the screen -- my son played with me. Thankfully we played on Mario Circuit from the SNES kart game, and I managed to edge out my competitors to take home the gold on my first online match!
In the end, the multiplay on Mario Kart Wii seems to be the best online experience that can be had on the Wii.