The Metro, is fairly clean, fast, and efficient. There is a slight problem with petty theft on the Metro, so keep eye open. The Metro is also the best way to travel from the airport, as well as the ferry port, it also services most of the tourist hotspots.
The buses in Athens are fairly clean, and efficient. The problem with the buses is trying to figure out the system. Google maps is a good source I've found for schedules and routes. Along with the Metro you can pretty much get anywhere in the city with relative ease using the buses.
The tram in Athens is more of a tourist attraction than a good source of transportation. The only reason you would want to take it is if you plan on visiting the beach. Getting from downtown to the sea via tram is a long trip; you’re better to use the Metro to meet up with the tram at the sea.
Taxis in Athens are fairly reasonable price wise, but really that's the only positive thing I have to say about them. I've encountered rampant overcharging, and many cabs reeking of cigarettes. If you do take a taxi from the airport, keep in mind that there is a flat fee from the airport to downtown. If you feel like you're being ripped off, tell the driver to take you to a police station to sort out any discrepancy.
As far as driving in Athens, my only tip is don’t, unless you have to leave the city.
Later