I really don't know if they treat AvisFirst members any differently - I get nice cars most of the time but not sure if it is because of AvisFirst or being on a corporate rate.

The only thing I know I get from AvisFirst is the free weekend coupon. Since my corporate rate does not allow me to get miles, it works out. The coupon has a few annoying restrictions:

1 - must pick up and return at the same location
2 - must have Saturday night rental
3 - blackout dates are happening again now the economy is better

Still, they do save a decent amount of money for weekend trips.