Wow. Just Wow. This game is amazing.
And everything they said that it would be was true. Gameplay, graphics, sound, and the online are all amazing. You can pick what character you want to be (like all Mario Kart games), AND you can pick which cart you want to drive. As you progress through the Grand Prix mode, you unlock more characters and more carts.
The Mission mode adds a lot to the game's life. There are 64 missions in the game to win, and some are pretty hard. The idea is to beat the mission as well as you can, to get the highest rating possible.
Online mode is super easy. I have a D-Link compatible router, so setting up the DS was a snap. I just entered in my WEP key, and eveything was ready to go. When playing online, the options are more limited then offline or LAN, but still enough to make online play VERY FUN. It can sometimes take up to a minute to find all the opponents, but hey, IT'S ONLINE MARIO KART!
This game is a must buy for anyone with a DS. Do it now if you haven't already.