Fun, addicting, full of variety; best Mario Kart yet.
First I'll start with the game play. It's very similar to other Mario Karts; turn, accelerate, drift, etc. But it had a different feel to it. It was just...more playable? Easier to get good at? Harder? I'm not really sure, but it felt great. It's addicting, and not just because it's fun. Unlike other Mario Karts, you don't spend just five minutes playing and become as good as you possibly can. This game required a certain amount of skill, and that separated it from other Mario Karts. It wasn't about what items you got; as you played you developed new skills and got a better feel for the game, thus improving yourself and enabling you to take on tougher challenges.
Another thing about this game was that it could get very hard at times. But that just added to the greatness! You needed to train and improve, and then come back and beat it when you're better.
And there where plenty of other things to do when the game got too hard. You could play the addicting and exciting multiplayer, control the difficulty in Versus, improve your item use and handling in Balloon Battle or Shine Runners, or play through the unique Mission Mode. At any rate, this game had a ton of variety, making its replay value incredibly high. You'll find yourself constantly coming back to this game, even if you've beaten it.
The kart election is very unique in this game. Not only does every character get his or her own unique selection of karts, but each one affects your performance differently. It's not that one kart is better than another, but rather that they're good in different things. So you have to find a kart that suits your personal needs, making kart selection important and in depth.
And the tracks were great; not the best in the series, but you got to give the Ds props; they look great, and are lengthy and detailed. Don't skip out on this game in fear of it's bad quality; it looks great.
I found this game to be addicting, fun, full of variety and lengthy. If you own a DS and are looking for a great, fun game, look no further; you do not want to miss out on this game.