I had buyer's remorse using them for a car rental last month. The rental itself was fine, we got an SUV for 8 days for $28.95 a day plus taxes/fees. But about a week before our trip, car rental rates on hotwire for the same dates dropped to $26.95 a day. I emailed them to see if they would match their own rate but they said no. I had no recourse since this is in their terms and cnditions. I think I'll stay away from Hotwire for car rentals unless it is absolutely last-minute.
By the way, we got Hertz and everything went well.