The apartment looks exactly as the photos show and more - it was very spacious, has 2 bedrooms and a large living space. Any amenity a traveler might need was also provided - including an iron/ironing board. Note: there is 1 bathroom and it is quite small.
Check in was fairly easy - we followed Konstantino's instructions and it wasn't difficult to find the place. His lovely mother received us, showed us around the house and explained how the appliances worked as best she could.
The apartment is in very convenient location (and seems fairly safe), a short walk to Monastiraki station and close to all the attractions.
The only reason why I'm giving a 4 star rating is due to the noise in the area - light sleepers beware! Due to the close proximity to bars/restaurants, the building's other tenants, blinds hitting against the glass windows, street cleaning, things being dropped from the apartment above, the next door's Greek Orthodox Church bell chimes and chants at 7am in the morning, it can be quite loud. Ear plugs work well enough.
Other than that, the apartment is really lovely and Konstantinos is a wonderful host.