The flat is central in the sense that you can get to the main sights (Acropolis, syntagma, Plaka) on foot (20-30 minutes).
The flat is extremely clean, modern, comfortable and very well equipped. It is very near the underground station of Thision which can be used to get further afield. The hosts are wonderful; checking in was done online, the keys were in a locked safe at the gate and they did not bother us at all. As a matter of fact we did not meet them but they were in constant communication with us via email. Anything I asked, I had the answer for within an hour. This was brilliant as I felt that they were there when I needed them and "out of the way" when I did not!
Parts of the area it is located are scruffy (grafitti, neglected buildings) but there are some wonderful restaurants nearby e.g Koudounakia, Zampano. Also for cakes, try Serbetospito! What an experience!
Overall, I would recommend this flat for being clean, convenient and well hosted.