I have a client north of PHL so I have to fly in there regularly.
Our company has a contract with Avis that includes the LDW for corporate rates, so I generally am expected to use Avis.
Thus, I use Avis PHL somewhat regularly.
Things you should expect:
1. The reserved class of car will be the class you receive, regardless of status. (I'm Avis 1st, with about 12-15 rentals a year specifically at PHL out of 50-60 per year for Avis in total.)
2. No cars (of any type) to be in the "Exchange" area.
3. The lot to be full of cars, including the "Upgrade" area.
4. The Preferred Desk reps to be generally assistive in upgrading your reservation if you proactively ask.
5. The return process to be pretty quick and painless.
Things you should not expect:
1. A proactively upgraded car.
2. A quick exit at the gate if you have a different car than assigned.
Best of luck!