The location of this apartment within Madrid seems quite close to perfect. First, you can take the Cercanias train line C-1 from the airport (terminal 4) to the Recoletos station, which is only a few steps away from the apartment. Secondly, the apartment is located in the nicest neighbourhood of Madrid, Salamanca, in walking distance of the nice shopping avenues of Serrano, Velazquez and Recoletos. And yet just on the other side of Recoletos, you have access to the cool areas of Malasana and Chueca, Gran Via and the city center. It is also beside the Retiro park, which is a gem, ideal for joggers and for those who want to contrast the hectic pace of the city with the calm of a classic and large park. The Prado is very close as well.
Though the short Calle del Cid is perhaps not the city's most beautiful street, the apartment building has perfectly reputable neighbours: on one side the chic Asian restaurant 'Le Dragon' and on the other, Spain's foreign assistance organization, the CDTI. On the opposing side of the street, a club for 'cougars' seems to have been around for a while.
The apartment itself is very well laid out. It is Ikea-furnished and equipped. The sofa and living room chairs show some signs of the passage of previous guests, but those are minor issues. The television has many channels and to see the movies in the original language, one presses the 'OK' button and right arrow until the choice appears (could be explained on a plasticized sheet on the coffee table). There is WiFi Internet, but it is a good idea to bring an Ethernet cable just in case your computer can't connect to the modem (as was my case).
The apartment is on the second floor of a 4-5 story building and the windows face an inner light shaft that is maybe 4 meters square, so the natural light that enters the apartment is mostly indirect. This might not sound ideal to most, however the up-side is that the apartment is quiet. One sleeps rather undisturbed nights on the twin bed.
In terms of supermarkets, the best is undoubtedly the one in the basement of the Corte Ingles on Serrano. An easy way to access it is to take the bus up Velazquez get off at Ortega y Gasset, walk to Serrano, do your groceries, and travel back down Serrano on the bus until the National Library stop which is steps away from the apartment.
The owner, Gabriel, provided good customer service.