We stayed at Nasos' apartment for just one night, before flying out from the ATH airport. It would have been a great place to stay for longer if you are exploring Athens. It is right by the Acropolis, and within walking distance of all the major sights in this city.
One nice thing about this apartment that I didn't see elsewhere in Greece was that the shower was inside a bathtub with a curtain, so water didn't get all over the bathroom floor when you showered, which was great!
Note when arranging a meeting time with Nasos that he does not live in the building, so he has to meet you at a pre-arranged time. If you get there early or he arrives late, then you have to wait on the street outside the building. If you rely on wifi to communicate, then you will have no way of contacting him when you arrive, so make sure you have an exact time arranged beforehand.
All in all, I would definitely recommend this place and would stay here again myself.