I stayed at the apartment for 3 days and my stay was perfect. The apartment is right in Plaza Mayor.
Host: Rocio was very responsive and accommodating. She met us upon arrival and responded to emails during our stay. A+ host.
Apartment: It was nicer than in the pictures. Quiet and clean, It fit 4 male adults without a problem. The AC worked great. The WIFI worked ok. The bathrooms were clean and spacious. It's within walking distance to everything we wanted to do, nightlife is steps away. Elevator in the building was convenient to take luggage up to the apartment. Luis (doorman) was very nice and accommodating as well.
Amenities: The washing machine was convenient and easy to use. Bath towels provided.
Recommendations: eat at El Tigre. It's a tapas bar that gives free tapas for every drink you buy. Buy a beer for 2.5 euros and get a large plate of tapas for free :) you'll basically get a whole plate of food for every drink you order.
I would definitely stay here again if I traveled back to Madrid.