We spent four nights at the apartment.
First of all, the host was extremely helpful. A representative has been waiting for us at the apartment although it has been quite late, and gave us thorough explanation regarding Madrid and answered our questions in fluent English. The refrigerator was filled with food, including kosher food which was bought especially for us.
The apartment itself is lovely. It is very cozy and clean, equipped with Microwave, many towels, and anything needed for the visitors.
The apartment is placed right by the heart of the Gran Via. We were a bit worried due to various comments regarding the street, mainly the comments stating that there are a lot of prostitutes in the street. These comments are correct, but as was mentioned by David, they do not bother the by passers in the streets. It should also be noted that all of the streets around the Gran Via are swarmed with prostitutes, and it is not something unique to this street.
Overall, we highly recommend the apartment.