I rent often from Hertz because of price and the AMEX 4 hour grace period. I spent a couple years as 5*, now a couple years as President's Circle. I haven't felt as a PC that I have noticed any consistent real difference over 5*. Well, maybe on 40 rentals last year the two weekend upgrades I got to a Mercedes-- one a sedan the other time an SUV-- is that about the PC advantage? About a 5% upgrade rate to an eye popping luxury vehicle?

Avis President's Club seems to be more consistent with some good upgrades versus PC. I rented a full size car last year for a month from Avis as President's Club and they upgraded me to a Cadillac for the four weeks which was some serious cost savings.