Our experience with Carlos' apartment has been really positive. The location is perfect: in less then 5 minutes you can easily get to public transport and a 24h supermarket.
The apartment is well equipped and the walls are quite soundproof, so you can barely hear outside noises. Moreover, the bed is really comfortable!
We met Carlos only once, but he has always been available and very kind.
If you want to spend some days in Madrid, his apartment's a really good place to stay in!