Jill -
We had a car on Crete. It was the easiest way to get to the ruins (Knosses and others.)
We arrived late in the evening. We met the car rental guy at the airport. He was in a hurry so we didn't get really good directions to our hotel (lots of narrow, one-way streets) so that was an adventure. My daughter didn't believe me about returning the car so that was a fiasco as well, but having a car was well worth it.
We were on Crete for 3 full days and loved the flexibility of a car. Once we were out of the capital, travel was great. Roads were well marked and we didn't have a problem driving anywhere.
Travel outside the bigger cities was minimal and we were there in July.
We did not have a car on Santorini, but took a day tour. Car rentals on Santorini were high. I think the bus transportation was fairly easy to figure out.
We did not have a car in Athens, but had one for 2 weeks when we toured the countryside. Once outside of Athens, car travel was great. The major roads were fairly well maintained and we didn't have too much trouble finding out way.
I'd taken an ancient Greek language class which at least taught me the Greek alphabet. Road signs are in Greek (first) then English(second). Have even minimal knowledge of Greek helped us make most of our exits/turns without having to back track much.
When we returned to Athens, my daughter didn't want to come in on the same road. Consequently, we didn't have a clue as to which exit to take. We ultimately ended up at the ferry in Pirius. Even though we had arrived at the ferry at 1 in the morning and had been taken to our hotel, we remembered the way and make it back to the hotel (right off Constitution Square) and just in time for the car rental guy to pick up the car.
We've travel to Europe 3 times and have always rented cars - make arrangements before you leave to get best rates.
MGS