Diddy Kong Racing DS doesn't do what Mario Kart DS does and it doesn't not do it very well either.
Well, this could suck. At its core, DKR is a solid racing game that has little gameplay flaws, the only problems come from the game built around those game mechanics. The overworld structure is clumsy and most of the tracks are very boring. Traveling from race to race seems pointless and tacked on. You drive to an area simply to be given what is essentially a race menu lobby. You race in races that have preset vehicles. You compete with a cart, plane or hover craft. In story mode you cannot switch between the types of vehicles for the vehicle specific races, which is all of them. IE, not planes vs carts, only karts vs karts and planes vs planes. Once you beat a track you get to replay it as a touch screen mini-game, collecting balloons and coins. The coins are used to level up your kart, hover craft and plane.
Here is where Diddy loses me. It is not made abundantly clear that to beat the second and third tier levels you HAVE to upgrade your vehicle. I found this out the hard way. Also, the amount of coins that it takes to achieve many/most of the upgrades is insane for the amount of time you will put into the single player mode. I don't see how Rare expects players to obtain hundreds of coins per vehicle to get certain upgrades. On top of that, you don't get the the coins that you collect in a race unless you actually WIN the race. I found myself just replaying the first 4 levels over and over again to to get enough coins to level up my vehicles.
The lack of more true Donkey Kong characters, Banjo or Conker, really makes the character line up dull. Not to mention the way the skill sets are made up, there are some characters with reason to ever be picked. Finally, and I know this will sound stupid, but DKR feels like you are playing a Nintendo 64 game. Mario Kart DS doesn't FEEL like a N64 game, it feels like a DS game. Maybe a graphical update or texture reworking would have fixed this, but overall it just feels old.
If you have Mario Kart DS, stick with it. DKR shows some interesting ideas in the upgrade areas and its fun to fly around in the plane, but you will find yourself drifting back to the wonderful and nearly flawless Mario Kart DS when done with DKR.
As a child I would have probably loved DKR, but I can see its faults now and wish that it had been tweaked a bit more before release.