I have been renting through The Rentalcars.com website several times per year for about 3 years now and have been very pleased with the prices I have paid. For example, when British Airways / Avis wanted £700+ for a 7 seater Dodge Grand Caravan in San Diego, Rentalcars got it for me for £250.
However, I am prompted to write due to two positive experiences with their Customer Service. Last December I had booked a car an Innsbruck airport through rental cars. The night before travelling to Innsbruck I printed out the paperwork and found the particular rental company would not accept American Express. I phoned Rentalcars and they managed to change my booking to another company that accepted American Express and were cheaper than the original one!
In May I booked a car for a week at St John's airport in New Foundland, Canada. Unfortunately, I accidentally forgot to pack my driving licence. I rang Rentalcars from Canada but they were unable to help me get a car without my licence. They told me to ring their customer services when I got home. I did this and within two weeks they obtained a full refund of £250 for me from the car rental company.
I do not take the extra insurance they offer because, being a regular traveller, I pay about £80 per year for an online insurance that covers all the rip-off excesses charged by car rental companies.