I have just returned from the RS Greece tour and wanted to let everyone know that Welcome Pickups is a great option for transport from the Athens airport to your hotel. The cost was about $49 and it was very convenient and the driver was easy to find, professional, informative, and I would use them again. My flight was two hours late in arriving but that was no problem as they tracked it and there was no additional charge.
It appears from their website (hard to find, but there's a list at the bottom of their "About" page), that they service areas all around Europe and beyond. I liked this much better than just grabbing a taxi at the airport. You even get an email with the name and picture of your driver a few days before the pickup. Most times I just take the public transport from the airport to my hotel, but for early or late arrivals/departures, I think they will be my new go-to.