Overall, my impression about the stay is 5/5. Some particular aspects of it will be described below.
Host: polite and accurate, communication is fast. The minor problem at the check-in was handled really quickly. Wine on the house was a nice touch. The guidebook was a real gem, which partly shaped my whole travel: I visited Vouliagmeni that turned out to be a picturesque peninsula, not just the ordinary beach. The bakery nearby, and restaurant Cactus-7 were also decent places to eat.
Location: it is just under Philopappou Hill, which is perfect for hiking and looking at Acropolis. Hiking to Acropolis itself takes about 15-20 minutes. The district feels safe, authentic, and simple.
Building: friendly neighbors without suspicious looks; good atmosphere; clean and modern space.
Apartment: the main advantage of it is surely the terrace, which is quite big. I'm a little sad that I traveled alone, otherwise it would be cool to seat there with friends and taste some wine. However, it's fair to note that it's noisy outside due the closeness of big road.
The bed was comfy and with warmers, which is a cool thing. The kitchen is fully equipped, some basic cooking was even undertaken. The bathroom is really, really small. Upon arrival home, my standard shower-room seemed a giant place in comparison to it.
Value: the price of about 30 euros seems to be a really good deal for such an offer.