This is hands down the best studio I've stayed in with Airbnb.
It's got everything you could want: it's central, thoughtfully furnished, wonderful views of the Acropolis... The space is small but extremely well laid-out, with a comfortable pull-out futon, all the fixings you need to make breakfast or pack yourself a lunch, and a nice workspace with good WiFi. Decently quiet at night for being situated right above a nightclub.
I stayed here solo for eight nights and did not feel cramped at all. Aris and Renata, the hosts, are extremely communicative, accommodating, and helpful. It's clear that this is a place they've put a lot of effort and care into setting up, and the payoffs are awesome.
If you're travelling alone or as a couple, this is the place to stay in Athens. Fantastic experience and highly recommended.