I don't have WiFi so I can't tell you about that, but if you love working on time trialing its a fantastic game. I was hooked for a while but I've been too busy to play recently. If you're looking for a good MKDS site where you can post your times try:

http://www.mariokart64.com/mkds/faq.php

Its a good site and they have a TON of videos to check out for strategies.

Don't pass up what might be the best game for DS.

Charging with Mario Kart DS is waiting for me to improve my times...