Belen's apartment is beautiful and exactly as described online. We were greeted warmly by her associate, Carmina, who showed us the apartment and the amenities. Belen was quick to respond to my questions throughout the entire booking process, which helped me decide on her apartment.
The apartment is conveniently located to the Royal Palace, Plaza Mayor, Plaza del Sol, the Basillica de San Miguel (small but pretty), and three Metro stations. There are also many nearby restaurants. They are a bit touristy, but the ones we tried had very good food (Amici had one of the best risottos I've ever had, and the Mercado de San Miguel is fun and lively with a wide array of food stalls). All these places are easily walkable, especially the Palace, Plaza Mayor and the restaurants.
I was a bit concerned about the directions provided to reach the apartment, but it was really very simple and none of the taxi drivers had a problem finding it. One tip I would give future visitors is that if you want to catch a taxi, go to the street below the apartment (turn left when you exit the building). It is a busier street and taxis come by frequently.
It is a great apartment and I highly recommend it.