This is a very good value place.
The location is probably the best: in the middle of the "triangle". Five minutes stroll from Monastiraki square and subway station, where you can take the train to/from the Athens airport ($10). 2 minutes walk from the central market. 10 minutes to Syntagma, Omonia, Psiri and its viviant Iroon square.
The apartment is in an art-deco building, with high ceilings an spacious rooms. A small balcony watching a pedestrian-only street some 10 meters away is a nice place to have a drink. One afternoon we had 3-4 rebetico musicians play on the nearby street corner, to our pleasure.
The neighborhood is not the quietest, and sound isolation is not the best, but we still got a good sleep as it was not noisy during the night.
Try the Turkish Lahmajoun place across the alley.