You should definitely book Nader’s apartment. It’s in a great, safe, and easily reachable location. It’s in walking distance to most of the main sights, and if you don’t want to walk then the metro stop is only steps away and easily connects you to anywhere you want to go. We bought the Madrid metro tourist ticket (unlimited metro/bus rides each day) which was well worth the few euros because we could just hop on/off when we didn’t want to walk.
The apartment was clean and exactly as pictured. Check in was easy and Nader was nice enough to allow us to drop our bags off early as we landed early in the morning. I would recommend this apartment to any friend!