Like Andrew said, the racing is a bit tedious, but I just have to get on and upgrade my rust buckets. I can't stop.
I just got myself a new 2002, and my Impailer is pretty much upgraded to the max. And I am saving up for more awesome classics and racing every chance I get (you know, at work when I have nothing else productive to do)
It's a pretty decent game for a phone so I suggest you waste time with it.