I tried it on a rental in Boston for kicks ... bid $90 for a two weekend day rental of a Jaguar (pretty much figured I wouldn't get it) or a Lincoln Town Car (figured $45/day for a Lincoln on the weekend wasn't that unreasonable) ... got rejected.

I don't think the deals you can get will be all that great, especially since you can keep bidding, even after you won, plus you can cancel the reservation.

For the "inflexibility" that is inherent in Priceline's stricted rules, I think that translates to a greater liklihood of a low bid being accepted.