The flat is very nice, well equipped and everything works just fine. It was also clean and had everything you needed for a stay in Athens - even a washing machine, which was important for me as i travelled with hand-luggage.
Nikos is a very nice and helpful host. The service was great, the communication very easy and reliable and he provided all important info about the flat during the booking process and also during my stay. He would also provide info about Athens (e.g. a strike of public services one day) that helped me a lot planning my program.
The flat is located very central. You could walk straight to the Acropolis, have Exarchia place around one corner and Omonia Square around the other. The Archeological Museum is right next, Taverns, Restaurants, Cafes, bars are very close, so it is an almost perfect location. Almost: because it is at the corner of a big street and the flat isn't really soundproof. I wouldn't recommend it to light sleepers honestly but that probably applies to everywhere downtown Athens.
long story short, I think it is a nice apartment for a short stay with light luggage in Athens - Exarchia.