Accessible, kid-oriented fun with outstanding gameplay
What's not to like about Mario Kart? It's built for fun from the ground up. This incarnation is still the staple of DS gaming three years after its release, and is the highest-ranking DS game on GameRankings.com (at 91.8%). Online play can get frustrating for casual players due to the overwhelming dominance of skilled snakers, but if you can't handle them then you might consider mastering the powerslide feature yourself, perhaps in single player mode.
Very well-designed tracks, loads of options (e.g. engine size) and unlockables (new characters and cars), excellent online capabilities, and even some advanced maneuvers for elite players make this as rich and compelling gaming experience as could be hoped for in this present era of video gaming.
Eventually you may tire of this game and never want to play it again, due to the cutesy kid-oriented design ethos that is inevitable in every Mario product. I've reached saturation point twice in two years, but I do keep coming back, a little more skilled in online play than the last time.
The prevalence of 'snakers' (what some consider cheaters who employ the game's boost system to drive about 20% faster than usual) in online play makes this a bit frustrating for the casual gamer who takes winning very seriously.
And for the snaker, it can be frustrating when everyone you play quits on you, and you are deprived of your win points -- although a clever player will keep their opponents connected by not getting too far ahead, and even letting them win a race or two.
The Bottom Line
The number one reason to buy a Nintendo DS. I probably played it for about a thousand hours before finally getting tired of it, and I am very finicky about what games I spend my time on. And, I am now back to playing it about an hour a day. Great game, endless replay value, overall an outstanding title and well worth the $50.
However, if you are not all that skilled at video games that reward reflexes, dexterity, and lightning-fast judgement, then you may want to start with Nintendogs or Tetris as your first DS purchase.