The guys offering the flat are the friendliest and most proactive hosts i had the opportunity to stay with over airbnb!
As for the flat itself, it is exactly as described in the posting. Clean, cutely decorated and has an awesome bed and shower. It was very easy to get a parking spot around the corner as well - which is rather uncommon for downtown Athens.
When it comes to the neighborhood itself please keep your expectations low. Neos Kosmos is dirty, loud and certainly not fancy. But if you are looking for a quick access to the center of Athens, it's absolutely perfect.