My wife and I have used AirTran several times in the past year, and I basically agree with Matt Wald on their tendency to have long lines at times. We haven't flown them enough to have any experience with their cancellation/rebooking procedures.
I would say they are a second-tier airline. Nothing wrong with that at the prices they offer, just not the service level of the "majors." I don't intend this statement as a criticism.
What I DO like about AirTran over similar airlines like Southwest, is they give seat assignments and manage the boarding process so you don't have to take your life in your hands during the boarding stampede. And, having gotten on the plane, you don't find 60 people already there ahead of you with everything from baby strollers to shopping bags crammed in every inch of overhead space. Also, unlike Southwest, I've never seen AirTran try to check in two flights departing at different times for different destinations at the same gate.
From where I usually fly out of, AirTran usually has a seat or two left even a few minutes prior to departure. This has saved my bacon more than once when ASA ("My Delta Connection") has yet again cancelled a flight or announced such a long delay that there was no chance of making my connection in Atlanta. I have been able to get to my destination and do so without paying over $1000 for a (literally) last-minute ticket purchase. Something to think of if they fly where you need to go.
I've blathered on enough. I don't have any problems with this airline. I would fly AirTran a lot more if they only had a better frequent flyer program.