In reply to Nic:
My experience car hire in Spain is this: ifyou want cheap, go with 'Ryanair' equivalents like Centauro, Goldcar, or some random agency you find on the 'net. You will wait 15 mins to be picked from the airport on some transfer bus, then driven 15 minutes to the car rental depot, which is offsite from the airport.
As for queues, it's potluck regardless of which company you use - I have friends who have visited us here twice, and hired a car from Goldcar; the first time they didn't queue at all, the next time they queued an hour!
If you want to 'drive away' from the airport, but pay 20% or so more for the pleasure, then go with one of the bigger companies (Hertz, Avis, Europcar etc). These companies also 'swallow' a lot more damage than the budget companies - I've not been charged for a shattered wingmirror in the past (with Hertz), but friends have been fleeced for small scratches with Goldcar.
My personal take is that, for a weekend trip, go for a bigger company. The potential loss of time with the budget companies is not worth it. But, for a longer trip (say a week or more), where 20% suddenly turns into 100s of euros, then it may be worth risking the cheaper companies.