YMMV
I have been PC for the last 12 years or so. No complaints and am a loyal Avis renter. IME, the upgrade experience vary's primarily by availability - and sometimes by location. PHL, for example, always used to give me a Cadillac, but over the past year or so, I am lucky to get a KIA, (no problem with this as long as the lost is not full of premium or luxury rides). MCI always goes above and beyond to acknowledge status and indicate that they gave the best they had a available. DFW pretty much lets you choose whatever you want except for the rare BMW. BWI makes an honest effort and is amenable to "swap" suggestions but can get very depleted based on rental volume. DCA can take a little more effort at the counter - but again I suspect this is capacity related.
I just wish they would bring back the Itunes certs. I stocked my ipod with over 1,000 songs back when that was an offering!