My experience at this apartment was great. It is newly renovated and I was the first person to stay there. It has all the appliances and supplies that you'd need for a short or long term stay. There's also a lot of natural light throughout the day. It's located quite close to Retiro Park, and it's about a 3 minute walk to the nearest bus stop. Walking to the nearest metro stops (Conde de Casal and Menendez Pelayo) takes 7-10 minutes.
The one issue I had was with the BUILDING, not Natalia's apartment. I missed my early morning flight because someone had double locked the building's main entrance the previous night. I had returned the keys as part of checking in before I went downstairs to leave. That was the first time the door had been locked like that during my stay there, so just make sure that the building entrance door is unlocked before you return the keys. Otherwise you may miss your flight and be stuck in the entrance hall for 2 hours like I was.