The apartment is just perfect, if you're planning on getting the full Athens experience. It's located right in the middle of the Psiri neighborhood, which is bustling and vibrating. The neighborhood has lots of awesome graffitis, cute stores and some nice cafés (if you explore a bit).
It's a short walk from Monastiraki square and subway station. The apartment itself was very clean, WiFi worked perfectly and it even has a nice terrace, from which you can see a part of Acropolis rock.
Communication with Bill (co-owner) was perfect. He helped where he could and was always available.
If you plan on staying during the winter months, I would advise you to bring an extra blanket though. It can get very chilly at night. Apart from that: perfect stay!