It's playable, which makes it the only playable racing game on the DS that doesn't include go-karts.
As for whether or not it is worth playing:
Plus points: Well the engine and physics are pretty good (a massive improvement on other non-mkds racers), there's a good sense of speed, the graphics are quite impressive and you can customize your cars in lots of absurd ways. There's ten or so cars to choose from including a Porsche, an Aston Martin, a Ford Mustang and every pikey's favourite the Golf GTI. Which are actually pretty fun to drive once you've bought all the upgrades, thanks to a decent control set up.
Bad points: Story mode is short and easy. The story itself is really really terrible. The AI sucks and has some of the most noticable rubber-banding I have ever seen. The only difficult part of the game is avoiding traffic which comes at you way too quickly but which isn't there in the 'boss fights' making those parts the easiest of the game (although it is possible to avoid the traffic with some practice).
Oh and the music... not really my cup of tea tbh, but no matter, some of the board members seem to like it. However, there is a real problem with it: There are only about 5 tracks in the game. So expect to hear them over...and over...and over again....
I'd say it is worth trying out, maybe fun for an afternoon and certainly the only racer worth playing on the DS right now if you - for some unknown reason - hate Mario.
EDIT: made it a little clearer