My wife and I have stayed in Athens many times, both in hotels or AirBnBs. This is my favorite lodging by far in our 10 years of coming to this city.
First the location is excellent. You are close to the metro, within walking distance to most of the fun tourist destinations in the city, and you can easiy get a cab at any hour of the day from Syntagma. The building is beautiful and is on a great street. The apartment itself is very modern, clean, and comfortable. The rooms are very large and having two on-suite bedrooms is great for a big group. The balcony is really nice in the evenings too.
Alexi was beyond accommodating. We messaged back and forth several times and he was very helpful, polite, and hospitable. We traveled with my parents, sister, and 8 month old, and everyone felt at ease and were beyond impressed with our stay here. If you're looking for a nice spot right in the middle of Athens, I cannot recommend this highly enough.