The apartment is such a nice (and clean) place to stay while in Athens. The apartment itself is quite small, but the big, sunny terrace more than makes up for this. Out there you'll be able to enjoy a full day of sun in what almost feels like your own private, lush paradise.
Walking to the center isn't too much of a walk or you can take the metro 10 minutes away.
I thoroughly recommend this place (2nd time staying there).