I highly recommend staying with Mari and Joe, they are so nice and the flat is amazing, too!!
I had a really good time (I stayed fro one week) and enjoyed our conversations. They also gave me clues about where to go etc.
Check-in and Check-out were flexible and the communication was fast and really easy.
The room is nicely furnished and decorated and feels a bit like home. Even though it's small there's everything you need. I had my own bathroom and it was no problem to use the kitchen. Everything was really clean. The flat is well-located in a quiet neighbourhood with lots of bars/cafés and not far from the centre of Madrid. If you need a place to stay in Madrid: Go there! :)