The stay in the apartment was a great experience and Jose Luis turned out to be a wonderful host.
I have stayed for over two months. The location of the apartment is just fabulous, literally one block from Museo del Prado and Royal Botanical Garden, about 15-20 minutes to Plaza Mayor and Puerta del Sol so really in the heart of the old Madrid. Despite its location it was a quite place (well, some neighbours could be quieter though). In addition to that the location is convenient regardless if you're a tourist or you stay for a business. Just few minutes away from metro station, large shop (Dia) across the street and lot's of typical Spanish bar around make it a place available for everyone.
The apartment itself is very well designed and excellently equipped, which is especially important for longer stays. You'll find there everything from A/C, dishwasher and washing machine up to microwave and oven. Separate bedroom with a huuuge wardrobe and lot's of cupboards in the kitchen will be sufficient for any number of stuff you might bring with you. Add TV and internet (well, it hanged sometimes but you will get easy instructions what to do then) and you've got all you need for your stay.
The set-up was more than enough for the two of us, and a folded comfortable sofa in the living room/kitchen part makes it easily suitable for 4 people.
In case of any problems you can always count on host's help, who even on vacation responded quickly and accurately. He was very kind, supportive and attentive during whole stay.
Note that even though there are large windows (with efficient blinds), the apartment does not get much sun due to it's location (it's 1st floor, facing the building yard) so it might sense a bit dark/shadowy there. Usually I was out anyway, either working or sightseeing but be prepared for that if you plan to be in the apartment during the day.
I definitely recommend this place and I will for sure use it if I'm back to Madrid.