This is going to be a super-long review, because I couldn't recommend this apartment more. Great space, great host, great neighborhood, very fair price.
This is a big apartment. I stayed for 27 days and as just one person, so it was a bit of a luxury home for me. But because Philio provides all of the amenities you could think of—pots and pans, oil and sugar, tons of clean linen and towels, lovely furniture, etc.—I think it also would be a wonderful place to stay just a few days. With a q/k size pullout bed in living room—you can close the door and turn it into a private room—and a q/k in the bedroom, the apt would be very comfortable for 2 couples.
The neighborhood is extremely safe, comfortable, and convenient. Don't be a bit concerned that it isn't in city center. It's literally a 1 min walk to the metro, where trains direct to Plaka come every 7 min or so. You could think about it as actually being just an 800 ft walk from Monstiraki and Syntagma, the main city squares, because you just step onto a train then step off.
Ambelokipi is the embassies district, so it's quiet and leafy, but you still have cafes, restaurants, grocery stores (yeah, plural), a barber shop within 2 blocks. You're next to the very famous luxury restaurant Balthazar: you'll see people dolled up and maybe a limo, and you can hear some music from the kitchen and the bathroom (*not* the bedrooms) till 3 am. But this is still a real Athens neighborhood, and your neighbors are almost entirely Greek. I felt like I became a real resident of Athens by the time I left.
Philio is also a truly wonderful host. She wasn't in Athens when I was moving in, but she arranged to have the key handed to me by someone else in person and introduced me to the apt over Skype. If I ever had a concern, she replied to me within maybe an hour. I'm also grateful for the great tips she gave me about the city (if you're staying a while, get a multi-day or month-long metro ticket, it'll save you loads).
With all this, per night it costs only a little bit more than Athens' nicer hostels.
Obviously I recommend this place.