Highly recommended!
I was visiting Madrid for the first time with a friend of mine and we both loved Mercedes' flat. The location is excellent; it is on a street with countless fantastic tapas bars, and it's within walking distance from all the main sites, the food market and the Sunday flea market.
Mercedes welcomed us when we arrived and was flexible with both the check-in and check-out times, which was greatly appreciated.
The flat itself is adorable. Everything was clean and as described on the listing, and there's an AC (for both heating and cooling). It was the perfect place to stay, and I would happily go back if I am visiting Madrid again in the future.