Sergio is a nice person, we were happy to stay in his apartment. The apartment is not big, but very comfortable and clean. We were in January, it was quite cold in the apartment, take warm socks with you :) There is practically no soundproofing in the apartment, but I think that is the case in almost all apartments in Madrid. There is good internet, you can work without problems. Excellent location smooths out all the small minuses of the apartment. Nearby within walking distance are many attractions, excellent restaurants and bars, supermarkets and shops. Perhaps you will have difficulty with parking if you are traveling by car, as this is the city center.
I recommend staying at Sergio's, I'm sure you'll be satisfied.