We stayed 2 nights in Carmen's apartment. It was a budget stay for us and it was appropriate for the price we paid.
The location is ideal from Atocha station (7-10min walk). There is metro nearby as well which was useful. The Prado museum is also at walking distance.
The flat is on the 6th floor and there is an elevator (although there are several stairs to climb with luggage at the entrance). The house is a typical guest house. The place itself wasnt the cleanest but we did okay for 2 nights. I liked how the room was warm during our rainy stay. There are 2 cats who loves to come into the room so if your not a cat person just make sure you close your door behind you.
Including Carmen, everyone in the house was welcoming and nice. The general atmosphere is very free and flexible unlike other places with strict rules to everything!
I would recommend this place for young travellers with low budget who would mainly be out and about exploring the city and just looking for somewhere warm and central to sleep.