In reply to RoyStone:
We've used various companies at Athens and all of them have been good.
The ones based at the airport are usually a bit more expensive but not always - We got an amazing deal from Athens car hire once (cheaper than everyone at the time) and they met you at the terminal with the car! When returning your car a good few of them (Sixt, Goldcar, Eurocar etc) are just before the airport terminal so it's dead easy to do the drop off and walk into the airport.(maybe 250m)
For those offsite they all run a pick up service which again we've never had any problems with and as far as I'm aware all pick up from the same area which again is very close to the airport terminal so minimal faff.
To give you an idea how easy it is.
Come out the terminal, turn right and walk (maybe 100m) to where the building ends, there is an obvious bus/minbus parking area in front of you on the right - they all park/pick up from here and the same on the return.