We can highly recommend Romanos, our host, and the place (Praxitelous 23 suite).
Our check in was smooth, and we were allowed to check in a bit earlier. At the check in, Romanos showed us the whole place (from bathroom to the balcony), gave detailed instructions about how everything works (there was also a manual for the suite!).
Romanos also provided us with a portable but pretty detailed map of the city center and supported it with oral comments and recommendations. They proved to be really helpful!
During our stay Romanos provided further help and support, swiftly answered our questions and requests. He helped us to arrange an early morning taxi to the airport when we were leaving Athens.
And moreover, Romanos is very welcoming, friendly and funny. It was very pleasant to interact with him.
The place itself is beautiful, clean and comfortable. Though it is a studio, it doesn't feel small, there's enough space for a couple. The bed is comfortable. There is a nice balcony. The suite has all necessary facilities, kitchen utensils (and also a compact grill that was great for sandwiches in the morning!).
Last, but not least, the studio is greatly placed. It is in the very center of the city, close to every place we wanted to visit. There are shops and great cafes nearby. I can mention only one drawback to the central location - the area can be noisy at night, and I sometimes used earplugs to get asleep. But that might not bother somebody else.
Overall, we had a great stay! If you want to stay in Athens - it is a great choice!