I stayed at Roberto's place for a short time. I really loved the neighborhood, it is one of the best areas in Madrid. Serrano reminds me 5th ave or Madison av in NYC, but it is less crowded. Best shopping stores, cafeterias, bars and restaurants. Lux brands and also cheaper brands, go to corte de ingles on Serrano (they have 3 stores just on calle de Serrano).
min to walk to Paseo Del Prado ( famous museo del Prado, Thyssen Bornemisza and Reina Sofia just right there), puerta de la independencia, Retiro park (saw peacocks just walking there), 15 min to Atocha Renfe train station (go inside), plaza de Cibeles, gran via, Sol and etc. Subway is literally 5 min to walk from the apartment. And easy to get to other areas of the city .
The room was nice and had a separate bathroom. Nespresso coffee. Host was super helpful and very nice person in general. Didn't feel weird at all :) Really had an amazing experience with Airnbnb, so much better than the hotel, and cheaper. Really recommend, you won't regret.