Simply a very fun game to play.
Ofcourse DKR is not a pure Kart game, but frankly that makes it all the better, the Hovercraft is pretty fun, and the Plane is loads of fun, and when the game came out the first time, it was something new, atleast for most gamers.
But unlike the previous version, this version doesn't have to pick up the slag from Mario Kart, as Mario Kart DS is a excellent game, but that doesn't mean you shouldn't get this game.
DKR DS hasn't changed much from the original version, the same races, some extra uses of the touchscreen and some other minor changes, but overall it's pretty much the same game, which isn't a bad thing except for the fact that many already bought the N64 version, and thus have to pay again for nearly the same game. This game is worth buying, but if you already own(ed) DKR on the N64, then you are probably better off getting Mario Kart DS first, if you haven't played it yet, then i would go for this game, and get Mario Kart DS later.