Great fun for all ages...
MKDS just blew me away, I still wonder how they got N64-like graphics onto such a relatively small machine. Making use of the DS' dual screens, you can race on the top screen, while on the bottom touch screen you can view a map. Pressing the screen you can change from viewing the whole track with players going round, or view the section of track that you are on. It displays it in so much detail that you race perfectly well by just looking at the map!
The game has a number of options which include the normal single player game, battle mode, time trial and online play. The normal single player experience is relatively short, and I completed it within about a couple of hours. Battle Mode can extend single player gameplay quite a bit more, so in all, I got about 10 or so hours out of the game by myself before I got bored.
But the feature that extends replay value even more is playing through wireless head-2-head or going online with Wi-Fi multiplayer. I myself have only played the wireless head-2-head with my sister, and we've had a lot of fun racing, and battling it out on the Battle Mode. For a complete gaming novice, even she could give me a run for my money, proving how well Nintendo design their games for both beginners and hardcore gamers alike. I would truly recommend this game for anyone of any age.
If you are thinking about getting this game, don't think again, it's an awesome experience, and you definately won't be disappointed. But make sure you try to get online through Wi-Fi, to get the most out of MKDS.