Great place to work remotely from Madrid: internet speed is great (~ 220Mbps download / 60Mbps upload).
The location is really good too, we found a 24h supermarket (not minimarket, an actual supermarket!) around the corner and there are plenty of places to eat, drink and go shopping nearby.
Our only nitpick would be the thin walls and the limited space, you can hear when everyone enter/leaves the building + there is a scholl literally in front of the flat. If you sleep soundly, that is probably not a problem though. The place is small but does have everything one needs for a short stay in Madrid, would recommend!