I have used DoYouSpain in the past, after a recommendation from a regular visitor to Spain, and had no problems at all. They were also the cheapest we could find at the time.
Never having used carjet before, as others have recommended above, I also looked up car rental in France for 2 weeks and nearly fell off my chair at the prices they were quoting. I have recently confirmed the hiring of an estate car for 2 weeks with economycarrentals.com, (I also used them some years ago) and when I ran the same dates and details with carjet, it was nearly three times the price. Also, with economycarrentals.com they are very up front about insurance costs (always read the small print in the t & cs) and confirm that there is no need to buy additional excess insurance at the time of booking as they will refund any excess you may have to pay in the event of an accident or theft.
I have no connection with them other than a fairly impressed customer.