The apartment is really well situated in the city. You can walk to many points of interest, including Plaza Mayor, Reina Sofia, Prado, Retiro Park and more. There are restaurants and cute cafes within walking distance as well and the metro is just a 2 minute walk from the apartment. The place is clean and very spacious for a studio. It is equipped with a washing machine/dryer combination that worked well. I am traveling to another city, so it was helpful to do a little laundry during my stay there. I felt very safe walking around the neighborhood, even late at night (11pm-12am or so).
The host is very responsive to questions. The apartment is run by a company, I think. I messaged with Begoña and was met by Marisela at the building. I was grateful that they let me check-in earlier since my flight arrived at 8am. They could not guarantee it, but allowed me to leave my bags at 11am (checkout time) at the apartment, and it only took the cleaning person about an hour to get my apartment ready.
The only big issue I experienced was the noise. The walls are pretty thin in this place. You can hear people above and across from the apartment. I don't think people are allowed to have parties at the building, but inevitably, people still do and it can get *very* loud. If you're a light sleeper, I would not recommend this place. But if you can sleep through pretty much any noise, then it's a perfect spot to stay and enjoy Madrid.