Public transportation in ATL GA.?

I am coming to ATL for about a week on vacation. I heard that the public Transportation system there is really good. I need your honest opinon. I have a rental car on reserve for the week but a lot of people said I should just catch the Marter or the City bus. What do you think. I want to go see everything and I do plan on hanging out late. How late do they run. How often do they run. I'm from FL. and we have an aweful PTS here. Please give me an idea and your opinion. Which would be best and Y. The car or PTS! Thanks yahoo'ers