I worked in Athens for a little over 3 months (September-early December) and stayed at Nick’s apartment for the duration. Here’s my summary about the rental:
The good: The apartment is conveniently located just a few minutes walk from the Kerameikos metro station, so getting around town is easy. There is a grocery store, bakery, pharmacy, a number of restaurants, and bars all within easy walking distance too. The washer/dryer is handy, and the apartment is well-stocked with linens, kitchen ware, and even toiletries. The TV gets BBC, which was nice, and the bed is comfortable. When I first arrived, Nick showed me around the neighborhood and helped me get a Greek SIM card for my cell. Generally, if you have a problem or question, he responds quickly.
The bad: It is a ground-floor apartment, located right next to the front door/lobby, and the walls are thin. You hear everything and everyone at all hours of the day and night. I found a few cockroaches during my stay. Nick told me (after I arrived) they are only a summer problem, and he did have the apartment treated once. However, the exterminator just sprayed poison around the baseboards, which I could do myself - and continued to do so while I was there. You generally have to wait 30-45 minutes to get hot water. The Wi-Fi is a bit unreliable.
The upshot: I think this is a fine place to stay for a short-term visit. If you are planning to be in Athens for more than a few days, check other options too.