The flat was fantastic and great, and I would stay there again if I visit Madrid in the future.
There were some check-in hiccups, but that was resolved very quickly. The flat was spacious and had all the necessary amenities. It's conveniently located - close to several restaurants and all the sites in Madrid are walkable.
In terms of the hosts - the lines of communications were fantastic and they were very responsive, before, during and after our stay. They even set up taxi from and to the airport. I would stay with them again!