Brilliant - the flat itself was spotlessly clean, spacious and bright. The location is very good too - you'll be about 10min walk away from a metro station (and only a few stops away from the centre of Athens). There are two very nice restaurants very close by - one of them is just downstairs, and both the food and the landlady are lovely! The neighborhood is fine; it's not unsafe, as some have said. It's a normal, residential Greek neighborhood, and walking at night with my partner I never felt unsafe.
Christos is a great landlord - highly prompt and responsive; absolutely no complaints whatsoever.