I bought the Nintendo DS version of this game for my 5 year old son and he absolutely adores it. We've collected quite a few (!) games now and this is still his favourite. It's very easy to play, simple controls and the game itself is simple. You race. Of course there's lots of different tracks, and cars you can unlock along the way, but it is just racing. It features the main characters from the original Disney Cars film, and as you progress you can unlock and race as them too.
I don't want to do the game down by saying it's simple. It holds your attention, and it's lots of fun, but I'm not sure how long it would hold the attention of an older child. And a more experienced gamer would probably complete the game very quickly. I'd say this is definitely one for the younger audience.