I just flew AirTran this past week. Flew from Boston to Ft Lauderdale, with a connection in Atlanta. I did pay for advance seat assignments..but paid for each way, not each leg. I did decide to pay for priority boarding since I was going with carryon only. Only one leg was it an issue. My flight from Boston to Atlanta wasn't very crowded, and I had chosen a seat in the back....no one around me at all, just FA's. It was sooooo nice. But, my Atlanta to FtLauderdale flight was much more crowded. While I had no one in the middle seat, it was hard for those boarding half way through the proceedure to find overhead space close to their seats!! I was glad I had paid for priority boarding. It's in zone 2 and you get to board right after the business class people.
For my flights home yesterday, again, no one in the seats next to me, on either leg. But, the flight from Ft Lauderdale to Atlanta was pretty crowded...again, not a ton of overhead space was available to those boarding towards the end.
I guess that if you have a lot of carryon (with a largish group of more than 3), I might recommend buying the priority boarding just to be sure you get overhead space. I believe it was $10 each way. Not a biggie.
I have to say that the times I've flown AirTran (to FLL) have been very good experiences.