I only flew AirTran once, way back in 2002, but I enjoyed the experience. The planes were in good shape (one of them was brand new) and the flight crews were both excellent. The price was also great, within $2 of SWA, who always has the cheapest fares from BWI-MCO.
Over the last 6 years, AirTran has also replaced every plane in their fleet with brand-new planes, right off the factory floor, so when you fly AirTran you will never be on a plane that's more than 6 years old.
I'd fly AirTran again without hesitation, except that SWA does not charge a fee if you want to change your flight, and I will often change my itinerary once or twice before a trip (adding a day or changing flight times or rebooking to get a cheaper fare), so I usually go with SWA.