Felix's apartment is in a nice secure building with a 24hr doorman in a good location in Chamberi. I picked up the key from the doorman with no hassles. It's nice and close to several metro stations, and also about 30 mins walk from many of the major attractions and center of Madrid. There are many supermarkets, bars, restaurants and cafes nearby.
The apartment faces the building's internal courtyard, so it's generally very quiet and you can't hear noise from the street. However it is also quite dark with not a lot of airflow, so be aware if that might be a problem for you if you're staying for more than a few nights.
The mattress is firm, but comfortable enough. The apartment has most of the things you'll need for your stay, but it's getting a bit worn and run down and could use a little TLC.
One thing to be aware of, there is not a dining table as the pictures show, so this isn't a good choice if you need a comfortable laptop working space.