Bit of advice....watch out for the hidden extras...insurance waivers, taking huge amounts for deposit from your credit card, etc.
Other stories I know of are when they take a huge deposit and (mysteriously!) your car gets damaged in a bodged break in then they take the deposit and if you have the waiver insurance then you can claim it back retrospectively but they have the deposit money and they are happy with that outcome. Usually turns out to be someone tipped off by the car hire company that is behind it and especially if they are aware of where it is parked (villa, hotel, etc).
Sometimes, it pays to organise the car from the UK and not out in Spain and take out the insurance waiver with it.
Sorry to be pessimistic but I know of quite a few cases of this occurring.