To start with the apartment is located at one of the best spots in the city (the very very center), from here you can walk to every highlight and there are a lot of good places to eat, drink and go out nearby.
The apartment itself is very clean and nice. It's more then big enough for two persons, a little bit small for four. The price/quality/location rate is absolutely 10 out of 10 point. I can not imagine there is a better place to stay in Madrid. And I did not even mention Alberto's hospitality and kindness.