PC treatment seems to be highly location dependent, but I've received decent upgrades throughout the US. Company policy dictates that I rent on a pre-negotiated rate with LDW included, intermediate or smaller, on a preferred provider (Hertz or Sixt). I've found when I rent outside in the US I generally get treated well with well trimmed cars even only renting intermediate.
Example: I rent intermediate at YVR and arrive. Letter from the manager thanking me for being PC, receive a Jeep SUV fully trimmed (panoramic moonroof, upgraded sound, 4X4 AWD, etc.).

Another anecdote: I rent at FRA, intermediate again. Hertz Carfirmation is for a Jeep Renegade. Flight gets cancelled I arrive a day later, get a Volvo V60. During my rental the manager emailed and called, offering that if I could get to the airport, I could switch to a Mercedes C series or BMW GT at no charge (I was far from the airport and declined while thanking them profusely for the offer).

Within the US upgrade experience varies widely (keep in mind I book intermediate):
- MLI usually put me in full size sedan, tending for lower mileage and touchscreen infotainment with bluetooth (they also noted my preference for vehicles with remote start and heated seats given the cold winters). I received a Buick on a couple occasions, but this is a small location.
- ORD upgraded me to a Buick about half the time. I also received a Mercedes once.
- IAH virtually never upgraded me. The two times I received upgrades beyond the obligatory full size upgrade, the cars had problems on interior (cleanliness or smoke smell) with very high mileage.
- BOI always upgraded me - if I didn't get a Buick Lacrosse or SUV (Enclave?) then I got a highly trimmed SUV. They were run by a franchisee and had fairly high rates ($36/day is normal for intermediate with LDW included as the prenegotiated rate, their rate was about $10/day higher).
- DFW - only went there once but they gave me a Mustang on an intermediate rental.

I've never been thanked as a PC within the US but I figure there's some sort of calculation going into the upgrade logic beyond purely being PC or the rate code.