We stayed at this flat for 2 weeks, and were really happy with our choice. The area is incredibly convenient and well connected - just opposite Atocha station, with many bus lines and the metro stop literally on the doorstep, the Reina Sofia Museum a few steps away and the Thyssen Bornemisza and Prado within easy walking distance. Supermarkets, many restaurants and the excellent Anton Martin market are all nearby, and it really feels one can get to virtually all the main attractions in minutes.
The flat itself is spacious, with a large living area and 2 ensuite bathrooms. Perfect for up to 4 adults + possibly 2 children (one needs to walk through the area with the sofa bed to access the main bedroom, so no privacy). It is a little dated, but in a way this is part of its charm as it sets it apart from the standard airbnb apartment.
It overlooks C. Atocha, which is busy, but one of the bedrooms is at the rear of the flat - and so very quiet, while the other is set back from the street.