We had a great stay at the Martin's apartment! Martin was easy to communicate with prior to and during our stay. The check-in/check-out procedure with Zoltan, apartment manager, was very smooth. He was very nice and helpful with all our questions and requests. He left us a botle of nice spanish wine :-).
The location is just great, 15-25 minutes walk to any major attraction in Madrid and from city centar and main squares. Palacio Real and Catedrala de la Almudena, Plaza Mayor, Puerta del Sol, Grand Via and all small streets of Old Madrid with a lot of great restaurants and tapas bars are at a walking distance. Mercado the San Miguel should not be missed.
The bus stop for lines C1 and C2 is in front of the building and 5min walking there is metro station so you can easily reach spots that are not near like Bernabeu Stadium or Caja Magica - we enjoyed tennis matches as Madrid open took place during our stay. There is a supermarket and Cebada mercado less than 10 minutes walk, which make it very easy to do your groceries.
We found the apartment cozy and clean, nicely decorated, and it had everything we needed for our 7 nights stay. It is evan nicer than you can see on the pictures. The neiborhood is quiet and calm. Going up and down four flights of stairs wasn't always fun, but at least gave us some exercise :-).
Overall, we had a lovely stay and would recommend the apartment if you want to have a great location for sightseeing in Madrid.