Not a bad game, but grossly overrated.
I know that people say that the online multiplayer gives this game lasting appeal, but I have to strongly disagree. It takes me a good 10 minutes to find and connect to players using Nintendo's WiFi service. If you're lucky enough to connect, you will have to deal with players dropping out of the game, unbalanced items ruining the race by completely changing the outcome, and the infamously annoying "snaking." Sure, I could "snake" too, but that makes the game more of a chore than an enjoyable activity.
In other words, don't expect more from Mario Kart DS than you would from Super Mario Kart on the SNES. It's the same game in a prettier package, with some annoying new features thrown in for good measure.