I have one of the '06 SS Impala's and I can't complain about it at all.
The gas mileage question - I got the car with just under 2k miles on it (demo driven by the dealership owner) and reset the MPG meter a few weeks ago. Driving about 90% city I'm getting 15mpg, getting out on the highway though it bumps the average up to around 20 something. I haven't had much highway time with it so I can't give an exact number.