My sister lives in Marietta. In October of 2008, my brother and I went down to Atlanta and we stayed at her house. When we went there, the weather was just perfect. Blue skies, sunshine, temperatures in the 70's, and not a cloud in the sky.
We were supposed to have gone down there that April, but when we got down to the Philadelphia airport, our flight was cancelled due to the bad weather down there. In fact, on that day alone, most of Georgia, including the Atlanta area, were all under severe thunderstorm warnings and tornado watches. Three weeks earlier (on March 14), an F2 tornado tore through downtown Atlanta, and there was a lot of storm damage. That's when I decided that we should go there in October. Thank God that the weather was just great then.
My father and my two other brothers went down there over the Labor Day weekend in 2003, and it was hot, humid, and thunderstorm threats.
In September of 2009, there was a lot of heavy rain down there, and some of the areas got flooded. I checked in on my sister just to make sure that she was OK, and she was fine.
The weather down there is just as the same as it is here in Philly, but I don't think their winters are as cold as the ones here.