Paula's apartment is perfectly located. Only a few minutes from Atocha station, where the airport bus stops. The art museums (Prado, Sofia Reina, Thyssen-Bornemisz), Retiro Park and good restaurants and cafes are very close by. Some Metro stations were closed for upgrading work whilst I was there but I didn't have to go too far to get to an open station. It is quite easy to walk to most places in any case.
The apartment is very comfortable and clean and I found the kitchen was well equipped. I found that I had to jiggle the key in the lock on the front door to the street to open but got used to it quickly. I would turn the key left and right until it turned freely and then opened the door. There is a light switch to the right of the front door when you enter the building and another one in the stairwell when you leave the apartment.
Apart from that I would only advise that you bring ear plugs if you are a light sleeper as Madridileños like to stay up late and it can get noisy sometimes. I slept soundly most nights though.
Thanks for the great stay Paula.