We had a fabulous time at Carlos' apartment. It is very conveniently located to many places to eat, and right in the middle of so many activities. It is the perfect central location. And although there is street noise below if the windows are open, with the windows closed you don't hear a thing, so it is not a distraction or any trouble. The apartment was very clean, and so much room to move around. There were just two in our party so we only used one bedroom, but it would be an easy place for a family of four or two couples. I highly recommend it. We walked everywhere - only needing the subway to meet a tour by the bullfighting ring. It is an easy walk to Sol or Opera subway stations.
Our only tiny hiccup was that I didn't have cellular service - only wifi - so I let Carlos know when I arrived at the airport and thought someone was meeting me at the apartment. Carlos had planned for a friend to meet, and that friend did not get the text I sent because I was putting +34 in front of it (like an international call - but now I was in Spain and the prefix was unnecessary). It was really our fault - but would have been helpful to have connected once we arrived at the airport and scheduled exactly. We have stayed in two others since then where there is a lock box and a way to get a key once in the building without having to wait for the owner, and that has been handy.
But none of it made any difference - we had an amazing time in Madrid and would stay at Carlos' place again in the future. Before our travels, he also communicated frequently and responded to any questions we had very quickly.