Rick's book mentions the 'homes for travellers' site (http://www.homesfortravellers.com/) as a source for apartment rentals. (It was also a link from MadridMan.com.) We have looked at a couple of them and heard back from the owners.
Have you rented in Madrid from these sources? What was your experience, either 'good' or 'not so..." Is there anything special to watch out for?