Anyone used Priceline for car rental?

I use it as well.  I've always had a big name company.  Usually I've had to hop on the airport shuttle to get to the rental car company (meaning rarely have the companies I've gotten been right at the airport, but they are still the companies for that airport code location).  I've gotten full size cars for around $14/day.  All supply & demand though.  Depends on where you're going, time of year, demand for the cars, etc.  as to how good of a deal you'll get.  You cannot just keep rebidding.  There is a window of time before you can rebid with the exact same specifications.  I can't remember what it is for cars off the top of my head.  Maybe one bid every 72 hours with cars?  With hotels it's 1 bid every 24 hours.  But I think cars might be longer.  With that said, you can usually change what type of car you are looking for (like add an additional pick) and rebid immediately.  I always start with the better cars so I can keep rebidding.  For instance let's say you pick this....

SUV $17/day.  And don't get it.  You can either wait out the waiting period and rebid at say SUV $19/day.  OR, you can now check another box so you're bidding SUV and Minivan $18/day.  (You could do $17 again or you can use this to go a little higher).  Don't get it, now check full-size, etc.  

I do the same thing with hotels.  I start at the highest star level, put in a bid, and then start decreasing my star level to allow me to rebid on the spot, or I'll add a second area I'm willing to stay.  I dont check everything I'm willing to get right off the bat b/c that's our chance to rebid.  Basically you only have to change one thing to rebid on the spot (hotel or car).   FYI...for hotels my bids that are accepted are usually LESS than 50% of the rack rate (or Priceline's published rate) and 90% of the time I end up in Marriott brand hotels when I pick say 2.5 stars or higher.  For instance if most of the hotels at a 2.5 star level are say $109/night, I bid $45 or $50 and it's usually accepted.  Pretty amazing.  Don't forget about priceline fees/taxes, but you'd be paying at least the taxes anyway.  It will show you your total cost with taxes, fees for that particular bid before you submit your bid.