Just used Goldcar in Mallorca. They were the cheapest by far, as evidenced by the massive cue at the Goldcar desk while all the surrounding desks were empty. BUT…..
They rape you senseless with the additional charges. With charges for an fuel, additional driver, insurance, and GPS hire (admittedly unnecessary), we ended up paying an additional 200 euros on picking up the car. The biggest rip-off was the fuel charge – 80 euros and you return the car empty. Except, of course, the fuel tank is never going to be completely empty when you return in (although I was tempted to do laps of the carpark until it was – followed by plowing into concrete pillar to make the most of the excess waiver that we paid for), and the Corsa that we rented could only fit about 50 euros worth of fuel in the tank.
Still probably marginally cheaper than other rental firms, but I’d much rather deal with people that are more upfront about the costs. Judging by the number of angry people at the rental desk, I wasn’t alone.