I've never used doyouspain, but a lot of people around here have booked through carjet.com and hispacar.com.
I've used each of them in alternate years for the past 5, and both have been reliable.
I checked doyouspain's prices on the car I booked this year and it's 4 euros less than my quote from Hispacar and 13 euros more than Carjet. So if you want to use a reputable booking agency, I can recommend either one.