I used to use Priceline almost exclusively for rental cars. And, I would still use them if I was able to get a good price.
However, the last two years, I have found their prices, while not "sky-high," NORMALLY, not cheap, either.
Indeed, my last four vacation rentals during the last year, Priceline never came close AS FAR AS I COULD TELL.
For example, I had a National Car Rental reservation for $108 (ALL TAXES, etc.) a couple of weeks ago. I made several bids on Priceline that would have been "around" $90-$95 and all were rejected -- and they recommended I bid about $150.
Of course, I may have been able to get a Priceline car for $100 or so, but with National, I can use my executive status, get points, not wait in line and get upgraded 80 percent of the time....
ALSO I have had Priceline give me a car only to find out later that the rental office was not open when I returned the car and there was no key drop box or anything else for "after hours returns." Calling National Corporate at 4 a.m. only got me the response of park it along the street and leave the keys in it, they will find it." This was at Ontario International Airport.
So, I really think you can get a better price on your own, but if you can't or don't want a hassle, I would try Travelocity first, as they seem to do very well on rental car prices. Once you get a price there, try to beat it on Priceline.