This flat is perfectly located for exploring central Athens on foot. We walked everywhere, from the Acropolis to many lovely restaurants, to the Metro station, etc. The location is RIGHT in the middle of where you want to be, I promise. The one caveat, like everyone else has mentioned, is the nightclub music that blasts until about 2 or 3 in the morning (yes, every day, even Sundays and Mondays). Bring earplugs. Also, if you're dumb Americans like us, don't forget to bring your electrical adapters!
Also, while the flat is quite large and spacious, it is also very minimally decorated. Like, it will feel more like somewhere to just sleep as opposed to somewhere you want to hang out, relax, and enjoy your surroundings. A good launchpad for Athenian adventures.