We loved this apartment on the Plaza Mayor. We could just open the balcony doors and watch all of the families shopping at the Christmas market, or go right outside our door to almost any place we wanted.
The Puerta del Sol is just two or three minutes around the corner, and from there you can catch the subway (inexpensive, easy to use and VERY clean compared to other urban subway systems) and go almost anywhere. It's only four stops from there to the Atocha train station, for example.
The Mercado de San Miguel is less than five minutes to walk, the palace is about ten, and the art triangle (Prado, Thyssen, and Reina Sofia museums) is about twenty minutes on foot.
We had one problem with the WiFi, but Noelia had it fixed the very next morning. Communication with her was excellent. She also arranged a taxi for us from the airport when we arrived.