Let me begin with the positives. The lines are well laid out and easy to learn/understand, including connections. Ticket agents speak English and are helpful (if there for several days, the 5 day Tourist Pass with unlimited rides except airport is a bargain and actually expires at midnight the last day, so I got 5 1/2 days out of it!). Cars are clean and nonsmoking, though it is virtually impossible to get a seat unless you're at the first stop or very young and fast. It runs until 12:30 AM Sunday to Thursday and a very convenient 2 AM on weekend nights, Fridays and Saturdays.
Now for the negatives. There is no air conditioning. While signs say to keep the windows closed since it is air conditioned, none of the cars had AC and we always had to open those windows to get any air at all. Throughout my week there, there were constantly announcements that "this line is out of service until ____" or "this concourse is temporarily closed" or "service is suspended on line such and such until . . ." etc., etc. The most problematic is the airport line, which runs very infrequently, supposedly every 36 minutes (I had the maximum 36 minute wait both times, leaving the airport and returning to it, unlucky I guess). The separate one way ticket they sell for this gives you only up to 90 minutes before it expires. So if you have the maximum wait as I did, 36 minutes, and then the 40 minute ride to Syntagma, that uses up 76 minutes right there. Then you need to wait for the connecting train, if it isn't one of the temporarily suspended ones, and ride to your final stop by the hotel and hope you don't exceed 90 minutes because if you do, you can't get through the exit turnstile. My first one expired and the second one I made with barely 5 minutes to spare. This needs to be looked into. Also, there are two different timetables for the airport one, a posted one at the ticket window and an online one. Agent said the window one was correct but the airport train arrived 15 minutes later than that posted time so I almost missed my flight. Improvements need to be made here.