It's an alternative Mario Kart DS.
What destroys DKRDS is the attempt at the DS functionality. Mario Kart DS delibrately left those features alone and it was all the more better for it. Not so here. The start up for a race for example doesn't work. You either rotate a tyre for the kart, spin the propeller around for the plane or blow for the hovercraft. It's awkward and YOU have to master it in order to keep up with the race pack who always boost off the start line. There's also several new DS specific puzzles that don't really fit into the context of the game. This game would have been great if they left all this stuff out. But no, they didn't.
The visuals are as bright and as colourful as you expect. There's no slowdown and everything is solid. It is generally pleasing to the eye and the music is just as cool as it was back in '97. A real eye and ear candy treat.
So okay, Diddky Kong Racing DS is not a bad game. In fact, it's a great game. But it has all these unnecessary features that dilute the pure racing that made Mario Kart DS a treat. There's content in DKRDS but you have to do a Lord Of The Ring style quest to get it all. There's the cool multiplayer Wi-Fi and local, a decent track editor (though all tracks must have this bland looking cloud background), a time trial and single race and a whole new adventure mode once you've beaten the first one. So yeah, the game is mind boggingly big but it's just not as fun as Mario Kart DS. Also, with the exception of Diddy, none of the other characters feel comfortable to race with.
I would say get this if you had your fill of Mario Kart DS. But if you have yet to play Mario Kart DS, make that title the priority.