Great fun, best played with others.
Probably the best part is the wide veriety of tracks to race on. They include both new courses and old tracks from all the Mario Kart games. Some tracks have been a simplefied though, you'll notice that the DS has sacraficed some things like background graphics and obsticles, but it's hardly noticable on such a small screen anyways.
The biggest change of the series is the use of Nindendo's Wi-Fi connection to play against players everywhere in the world. You'll get much better competition then the computer that always puts the same racer in any one series as your main advisary. Real players are more fun and offer more veriety to the game and will definetly keep you on your toes. Some players are novice and some are extreme experts that powerslide most of the race to gain boost, a method coined 'snaking' because of the zig-zag motion the racer makes.
But the biggest drawback to online gaming is the long wait periods it takes for Nintendo to match you with opponents. Whether you select regional or worldwide you'll often wait a few minutes while online players are matched up. After opponents are selected each player selects their driver and kart, which are unlockable in the normal game. After drivers and karts are selected you may then choose a course to race on. The computer will randomly pick one of the tracks that everybody has voted for unless there are two or more votes for the same track. After all this you still end up with players disconnecting during races or doing other unsportsman like gaming, such as staying in last place with the 'bullet bill' item on purpose and then using it to zoom into first at the endo of the race.
However, a fun game and deffinetly one that will be enjoyed among friends whether battling to keep balloons or in races.
Perhaps another drawback of online play, no battles, only racing.
Over all a great, I just chose to pick out my biggest dislikes of a great game.