The game looks great and the racing is fun, but you can't seem to mod your cars out. That really takes away from the feel of the game for me.
You can't mod them as such, but you can play around with the tuning. You can adjust almost everything, acceleration, gearing, brake strength, ride height, turning radius, suspension etc. The only thing I find annoying, is the penalties for taking short-cuts, hitting walls and other cars. Otherwise it is an awesome game.