I like the C6R. It's just what you'd imagine a GT car to be, fairly decent through corners although it can get away from you very easily if you're not careful. I like it but I don't race it very often. I think it's one of the better cars on the NTM however.
C6R is a great car, you have to very careful though. It locks up its brakes very easily (at around 60% of braking force in higher speeds) and it's a bit wobbly in corners if you push it too much. Minor mistakes can be corrected, but press the throttle too much and you'll definitely end up in barriers. The more I drive the more I am able to save it from slides, but sometimes it's not even humanly possible.