My wife and I had an excellent time in Madrid and our stay at Carlos's place was great! First, Carlos was an exceptional host. He was always quick to respond both before our trip and during. He offered great recommendations for things nearby including where to store our luggage while we waited to check-in so we could start exploring the neighborhood right away.
The flat was clean and well stocked. The second bedroom was the perfect place to hold all of our luggage so it was out of the way. The bathroom and kitchen are small but was not a problem for us. The temperature was very comfortable and the ceiling fans in each bedroom helped keep it cool at night.
The neighborhood is away from the busy tourist spots while still being conveniently located to public transit and an easy walk to things like El Rastro, the Prado, El Retiro and the Royal Palace. We were able to easily reach almost everything by foot or public transit and for anything else we use ride share, which is much less expensive than in the states.
Overall, I would highly recommend Carlos's place for your trip to Madrid!