Comfortable location, both for visting museums (Prado Museum and Reina Sofia art gallery are a five minute walk away) and to get to the centre of the city (subway line n°1 - Atocha Renfe or Menendez Pelayo stations are close to the apartment).
Mercedes is extremly friendly, charming and disposable (and her friends too)!
The house is beautiful, modern and clean; the bedroom is a little bit small, but comfortable.
In conclusion, it's an ideal base for visiting Madrid... absolutely well-recommended!