I just looked at my last 12 months of Avis rentals, and compared what I reserved, which was Intermediate, with what was rented.
8 rentals were made with Preferred status and an upgrade of at least 1 class was waiting for me 4 times. Another 2 times it was an SUV, and 2 times it was what I reserved(no upgrade, and I didn't ask to change).
The 4 rentals I made with Presidents Club status resulted in 1 Luxury and 2 Premium cars without having to ask. One rental had a SUV assigned and I asked for and got a brand new compact that I requested because it was a new for 2012 model that I hadn't yet driven.
I'm happy that I get an upgraded car frequently without having to ask.