Echolynch,
Happy for you that you had better luck than I did, but am guessing is was at the Rental Agencies discretion (independent of Priceline) and since the overwhelming majority are franchises, experiences can vary widely.
Pricelines "name your own price" policy states: "Rental cars are non-refundable, non-transferrable and non-changeable even if the reservation is not used.
It's been awhile, but there was a "Venting thread" here on TA wherein, as I recall, the traveler took an earlier than scheduled interIsland flight, arrived at the counter and was told (despite the fact that there appeared to be numerous available cars), "you have to wait until the contracted scheduled pick-up time" (have zero recollection if it was Alamo). Had it not been a pre-pay, it wouldn't surprise me if the solution would have been to simply walk to the competition next door.
Don't pretend to know the intricacies of the business, but perhaps (probably?) Priceline rentals are lower profit-margin to the chagrin of some (many?) Franchisees who are forced to comply with Parent company acceptance policies whether they like it or not...and there may be plenty of times in popular tourist destinations where there is local "who needs Priceline anyway?" resentment.
It's also been awhile, but seems as though many Kipplinger/Smart Money etc. articles on Priceline include a horror story of a completely cancelled vacation with 100% forfeiture of the cost of a pre-paid car (or Hotel) rental.
Guess it sums up as another case of "no free lunch"; get a lower price/accept higher risk/pay your money/take your choice.
Far as I'm concerned (other than relay of price from Discount Hawaii) enought time spent on this issue; so if the last word is a priority, be my guest.