This apartment is in a great/ diverse neighborhood - close to Lavapies and Embajadores metros. However, Madrid is a "walking" city. The location of this apartment is walkable to just about anywhere- Prado Museum, Retiro Park, sol, Plaza de España.... just about anywhere. The metro is literally about 2-4 blocks in each direction from apartment.
It is great for 2-3 people....no more than that. It is a walk up, so be prepared to climb some flights of stairs. There is a lot of noise outside at night with people hanging out in the plaza and surrounding areas- you can hear it all. It is not ideal for light sleepers.
The place was very clean and host was very responsive when I wrote to ask about using the washer machine. However, I would recommend a book explaining how to use things in the apartment- stove, washer machine, etc.
I would definitely return here with friends on another trip!