Guillermo was a really nice host and we had a fabulous time in Madrid. However there were issues with the place. The room was dark and cold, and the bed really small and uncomfortable. The kitchen was often messy, with rubbish overflowing from the bin and a lot of smelly food. The bathroom was decent, however the towels were old and had an unpleasant smell.
Guillermo's flat is well located, and the room would probably be more suitable for a single person, who doesn't expect to spend any time in, and perhaps is in Madrid on a short visit. If you're a couple, looking for a cozy place to relax after a day of exploring the city, I would look elsewhere.