Our stay in the cueva was okay. It's small and it was a little cold when my girlfriend and I were there early January. When we booked it we didn't realize there's no internet, so take note if you need it. Everything was in one room (aka toilet, shower, kitchen, bed). The bed was just a mattress on the floor. The toilet, being in the same room, made it embarrassing to always go to the washroom in front of each other - if you're a couple or two friends, take note of this. We also didn't have any toiletries or towels, but maybe now they are provided.
The small fridge was a good size, Olga pushed her food to the side to make room for ours. We initially picked this place because we wanted a kitchen and we didn't want to go a hostel. In retrospect, we think we would've preferred a hostel and wanted to leave after a day, but we knew we probably wouldn't have gotten our money back - we did leave a day early, but only through paying Olga 10 euros. Olga was nice - when we came to the place she explained how everything works. She doesn't speak English, just French and Spanish so make sure you know some basics in one of those.
The location is excellent however - close to all major attractions in Madrid. If you're looking for the basic and don't need internet then this place works.