Javier is an amazing host - super responsive and ready to solve any problems visitors might have. We checked in on Christmas Day of all times and Javier was there to meet us and took the time to really explain everything before leaving for family obligations. He checked in a few times to make sure everything was to our liking and was ready to solve any problems that we had.
The space itself is wonderful, though not huge- for the three of us (me, my wife and our 2 year old son), it was the perfect size. Javier provided a few water bottles and milk in the fridge as well as coffee pods for the coffeemaker. The bed was incredibly comfortable and we slept really well - the street stays relatively dark in the mornings (though some of that was the season/weather). Given the time of year and the temperature in Madrid, we especially loved the efficiency of the heater and the nice, hot shower.
As far as location, it's hard to imagine a better spot. The studio is on a quiet street that is nonetheless an easy walk to the Prado (maybe 5-10 mins), to the main plazas or to the palace (20-25 mins). It's maybe 100 yards from the Anton Martin metro entrance, so transfers to/from the airport or the train station is quite easy. The neighborhood is mostly made up of tiny, one-lane streets that don't see very much traffic at all, even during the day, so it's really pleasant to walk around. There are several good restaurants and bakeries nearby as well.
Overall, we really loved this studio and our stay there. It's a great location and a place we loved coming back to each day after sightseeing. Javier is a great host and his flexibility, particularly given our check-in day, was greatly appreciated.