Z4 - absolutely awful sound it put me off driving it. But the feeling of this car is alright, nothing more.
R8 - probably the best handling car of the lot, because the thing moves out quite a bit. Probably moves a bit too much under braking and off throttle. The best sounding car.
SLS - absolutely atrocious physics. If this was on GT5, it would be the worst handling car in the game. You can't get this car to get out of shape at all. On throttle, under braking, nothing. Sport softs on the back makes it handle much better, but still not as loose as the R8! (that's how bad the SLS is). The sound is... not an SLS. They're better off just using the road car sound sample.
GT-R - similar physics to GT6 demo on PSN store. How the differential is set up is stupid. Outside tyre spins up way too easily.
In terms of lap times, the R8 is seconds faster than the other cars. I think because of its MR drivetrain. The SLS is slow. If they don't fix them in time for launch.....