After a few unconclusive rentals following my Avis First enrollment
I can say that things have changed ...
I always book a full size with my corporate rate and for most of the past 25 rentals (since Avis First) I had a nice upgrade waiting for me :
several Towncars, several Mercury Grand Marquis, many SUVs/Crossovers (Ford Edge, Suzuki , Ford Escape, Dodge Nitro, Jeep Commander, Trailblazers, Explorers)
On the occasion I get an Impala it's always a fresh vehicle with no more than 7000 miles ...

Now I wonder what is the next level above Avis First and what would be the perks ?!?!?