Just picked this up today, and it's absolutely blinding.
Perhaps the best version of Mario Kart ever - though the original SNES game is pretty damn near perfect - and the Wi-Fi connection works beautifully. My wireless router is already set up for my laptop, and it was a matter of two or three clicks & a quick connection check to get it sorted on the DS. A few minutes later I had the game up & running and was playing with another pair of players in Worldwide mode. No lag, no hiccups and I've played through several Wi-Fi championships now with no problems at all (well, apart from being so rusty that I got my arse tanned!)
It's an essential purchase for DS owners, and I'd highly recommend it anyone else with the cash spare to pick up the game & console bundle that's out today.
Roll on Metroid Prime: Hunters and Animal Crossing!
aahh- that brings back memories of playing Mariokart on the snes too(I bought it second hand, and at a time when everybody else was switching to playstation*, but still..)
*incidently, favorite game on that- Hogs of War.