Wayner -- that Metro ride, while convenient, is also the #1 Ground Zero for pickpockets... who target newbie tourists who carry money & credit cards & passports in their pockets (especially men -- they say "I'd KNOW if anyone touched my pocket!!" and 75% of victims are men), or others who carry them in backpacks, or in unzipped daybags. These cars get crowded as they approach the 2-3 main stops in central Athens, and then these (very talented) operatives start a distractiion , by bumping/dropping stuff etc, then whoops! they're out of the door. Newcomers, burdened with heavy bags, don't realize what's happening until too late.
Even if one is super-vigilant & escapes the pickers, when the Metro stops at for instance, Syntagma or Monastiraki, a huge mob is trying to exit, while another group pushes to enter, and people with heavy luggage struggle to get out before the doors close. Then they are faced with walking down corridors, taking escalators or stairs, and at Syntagma, even when they emerge into daylight they are way below street level, must pull bags up a ramp.
For this reason, if taking public transit from airport to save a few bucks, I vote for X95 express bus to Syntagma; it is right outside the airport door (follow the signs) u buy tix at a kiosk (€6) and swipe electronically as u enter bus. They go every 20-30 minutes 24/7. Big bags are piled in a central pen, very safe; you keep your shoulder bag with you. Comfy seats, no one can board after leaving airport and only 2-3 stops for exits, so pickpockets can't operate. You can even doze or read during trip... except in rush hour, it's just about the same or 10 minutes more than the Metro. AND when you reach Syntagma Square, that's end of line, so no rush to de-board; take your sweet time, and step right onto sidewalk. My hotel is about a 10-minute walk downhill on Amalias Ave, so I just pull my rolling bag. If one's lodging isn't that convenient, one can get a taxi for a few €€ and still save big over a solo taxi ride €38.