I've had good deals via Priceline bidding whereas direct booking via rental companies for the same time period was extremely expensive. For example, last summer, for an ORD trip, I couldn't find anything for less than $60/day via Hertz, Avis, Budget, etc. With Priceline bidding, I ended up with an Avis car at a rate of about $25/day for the same car type, dates, etc.