The apartment is truly wonderfully located, both in relation to the airport (a couple of minutes walk to the Atocha station, from where the direct airport bus takes you to and from the Barajas airport in about half an hour) and to most of the main sights. We had no need to use the bus or the metro in all of our 5 days in Madrid, choosing to either walk or use the very convenient public electric bikes. There is plenty to see and do in the neighbourhood as well, lots of little cafes and restaurants, making it a lovely area to stay at. It's also a lively area with a karaoke bar right across the street and plenty of people in the street at night, but as the other reviews mention, the noise is minimal with the windows closed and we had no trouble sleeping.
The studio is also beautifully decorated and perfect for a couple. The reality matches the photos, which is always nice. This is one studio (out of four in total) in a large flat that seems to have been divided into smaller units, they share one door and a very small hallway, but each has its own keys and we had no issues with other guests. The whole flat had some unexpected electrical issues (the power went out) on two occasions during our stay, but Maria was quite responsive in the situation and the issues got fixed in about an hour or less each time.
The only downside is the damp smell that was present in the apartment when we got there and it would be there when we got back to the apartment in the evening every day, but then we would open the windows for a bit and it would go away. So all in all, not a huge nuisance, but if you're very sensitive to smells, it might be an issue (of course, it might be a temporary thing as well).
Other than that - awesome place and we were happy to call it home for a couple of days.