The apartment of Juan is simply amazing!!
Even if we didn't met him directly, he sent us all the information about not only the apartment but also about all the things to do/visit in madrid.
The apartment is very modern and it is situated in Chueca. This neighborhodd is situated in the center (5 minutes on walk by plaza the cibeles, 10 by gran via..) but nevertheless is extremely calm!! There are lots of bar, pubs, restaurants that are very characteristic and everyone is nice.
I strictly recommend this apartment! we were four and we stayed there for 5 nights without any problems of space.
PS. The only thing is that it is in a building without the elevator and the apartment is at the 5th floor. (but it should be my mistake.. maybe it is written in the characteristic of the house and i didn't read it)