Addictive as hell. Best Mario Kart yet.
What has really struck me about the game is that it really does have a near perfect difficulty curve, even with just three different AI levels. You'll find yourself thrashing the 50CC GPs on your first play, but then having to try 10 times just to come equal first in 100CC. However. what it does do is make you want to play it all again. You'll slip up once on one race, costing you a valuable position, but then straight away you'll jump straight back into the GP from the start, it's perplexingly addictive.
If you have friend or relatives with a DS there is absolutely no reason not to get this game. It offers what is truly the greatest multiplayer experince offered from a handheld game of all time, even 2 or 3 players can be insanely competetive and fun (which is added to by the incorporation of bots to fill the number up to 8). And I can say that without even having playing it online (yet). The only fault in my opinion is that it can get laggy in multiplayer when in Battles or with all 8 people in close contact, with bomb-oms and the like going off. Even if you can't get it online, it's still one of the best games on DS. Buy it.