If you are looking for a good start on iPhone racing games, I would recommend Fastlane Street Racing, although it's not really on sale, since it's always been $0.99. The game was pretty impressive IMO, the graphics are solid and it got a lot of tracks to play on. Need for Speed Undercover is on sale for $2.99 at the moment, it's pretty much the best racing game on iDevices right now if you don't seek multiplayer mode.
Real Racing is good. Haven't tried Asphalt 5 personally, but worth a look. Has online multiplayer I believe.
I love NFS right now. I'm not even a huge racing person. In fact, the only racing games I have ever gotten into were Burnout, Mario Kart and Twisted Metal (if you even call that a racing game). But NFS has kept me coming back over and over. Otherwise, if you want just a good driving game but not really racing, check out Jet Car Stunts.
