Hard to find a better location to stay. The flat is in a quiet street twenty steps away from shops and restaurants in Plaka, but also very close to Monastiraki and to the National Gardens. Several places of interest are at walkable distance; for the others, Syntagma and Monastiraki metro stations are 7-8 minutes away.
The flat is rather big for 2 people, and pleasanty cool in the summer. All we needed was available in the flat. (We did not use the kitchen, as the restaurants in Athens are cheap and very good, so we cannot comment on that.) Especially for the Italians out there, there is even a little shower than can be used as a bidet. If you have a very light sleep, you might consider briging earplugs with you, as you might sometimes hear the neighbours in the morning. But otherwise, the street is really quiet.
Finally, all communications with Christina were fast and smooth. Highly recommended. When we will be back in Athens, we are definitely going for this flat again!