Any advice on transportation from Athens to the Airport?
Taxi
There are a couple public options, the Metro extends to the airport, though in the past it was on/off due to construction. The traditional option is the express bus from Syntagma Square, runs 24 hrs.
Metro. It was working fine when I used it in 2006. The Athens metro is newish, clean and easy.
If you have access to a metro station, take the metro. It will zip you right to the airport terminal building. As Kathy says, it's clean, modern, and efficient.
We have used the bus recently from Syntagma Square and ii is easy and efficient. The biggest advantage is the pick up point on the square. We were staying within walking distance of the square but to use the metro it would have been a long walk through the metro station to the train.
We just left Athens 4-21-11 and our hotel was within 3 blocks of Symtagma Square where you catch the metro that took us straight to the airport. The metro was cheap and a straight shot.