They are fast, quite frequent and available for 5:30 to midnight (2:00am on Fridays & Saturdays)
The price is quite low (especially if you compaire it to other European capitals' subways): 1,40euros lasting 90 minutes. Of course there are daily (4euros) and weekly tickets (14euros).
For more information please check: http://www.oasa.gr/content.php?id=tickets&lang=en
Buses are the next best thing. However, to be honest when traffic hits you would prefer to be underground...
Taxis are for emergencies only! There are some good taxi drivers (approximately 5% -you do the math!). Especially for getting from and at the airport do prefer to use the suburban railway (8euros for one person, 14euros for 2, 20euros for 3).
Whenever you enter a train or bus always be extra careful about your personal belongings!