The apartment was great and we have no complaints whatsoever. Our airplane arrived well before the apartment was scheduled to be ready, but they allowed us to leave our bags in their office so we didn't have to drag them around town with us the first day. Everybody was friendly and helpful before and during the stay.
The apartment was as advertised. It was large, comfortable and well stocked with toiletries, condiments, cookware, appliances, etc. The only items we bought was extra toilet paper (we stayed a week), bars of soap and shampoo. The apartment was bright and sunny though there is no view to speak of through the windows.
A few minor things didn't work quite right, but we didn't report them until our stay ended since they were minor. However, they kept in constant touch with us through email to make sure we were satisfied. No complaints about their service.
If back in Madrid, this place would definitely be on my list to stay again and I would recommend it to anyone.