The place is very nice! It is exactly as described on the airbnb website and the location is perfect. The air conditioner, washing machine, microwave, etc all worked fine. There is enough cutlery, pots, plates, etc. For cooking there are two hobs, a microwave and a kettle (no oven though). The building was clean and safe. The place is relatively quiet, especially after midnight.
I didn't meet with Javier, because it was a working day when I arrived, but the porter Ivan met me, gave me the keys and showed me the place (Javier told me in advance that this is the case). Everything worked fine with checking in and leaving. In the flat there were also lots of maps and books full of useful tips for tourists in Madrid.