I don't have enough experience with Priceline, but for me -- which may be a typical leisure traveller pattern, my rentals are usually:

Intermediate or Full Size
Two-Four Weekend Days

I am usually able to, very easily, get $20 a night, including taxes/fees on one of the major car rental companies. So $40-$60 for a typical rental. If I want to save at least $10, then that means my bid would be $30-$50, or subtracing the $6 PL fee, $24-44 or less than $12 a day including taxes/fees.

The math just doesn't work out for me most of the time. But sometimes it has -- I have saved over $30 on a full size rental in PHX once.

Also, these days I have 'fast car' access at National, Hertz, Budget and Dollar. This fast car access can make a real difference, so I've rarely even thought I can make enough of a price difference to bother.

Just my $.01