I stayed at Mercedes' apartment for a month and overall was a good first airbnb experience.
The apartment itself is just as shown in the pictures. Spacious, well equipped and with tasteful decoration.
The location is very convenient to get around the city and the neighbourhood is quiet and pleasant to walk around. You have some of the key attractions of Madrid at a walking distance (Parque del Retiro, museums such as Reina Sofia, Prado and Thyssen, and Paseo de Prado, to name a few).
As for the host - Mercedes, she is a friendly and outgoing lady, who you can have a nice chat with and will give you some great tips about Madrid from tourist attractions, pubs, cafes, etc.

Thank you Mercedes!