We stayed in the apartment for 5 nights during the holiday season.
The apartments is completely renovated and updated with a home feeling. It would be prefect for a family of 4 with an option of another person on the living room couch. It is located very well away from the crowded center but within a walking distance to all major attractions and if it is raining, you can walk to the metro station which is 2 min away. Sunday mornings could get noisy a little as the flea market is right down the street (great antique stores all around!) Building is renovated with a nice small elevator that was never busy (a big plus).
Marta and Stephanie were very accommodative and responsive and we would definitely recommend to stay there once you are in Madrid. Thanks Marta.