When I've booked through PLT travel services the daily rates the quote me sound high BUT the quote an average price per day rather than X weekdays @$ and X weekend days @$. When I compare that to a quote from the car rental people directly they usually end up very very close. I don't use priceline type bookings so I don't know that comparision.
I would personally use the certificates when renting a car during high dollar times when there are NO good deals. We like to use them when we are renting a 4WD suv from Denver at spring break or Christmas. Rates are sickening then so the certificates help!