I used to fly Airtran (I go out of BWI) but I am using SWA from now on. We flew Airtran this past May. Our flight was supposed to leave at 8:40am, well 5 mins before we were supposed to board, they announce the flight is delayed b/c a part of the plane is missing. After an hour, they annouce the part still hasn't arrived. After about another hour, they announce that the part is here, but the mechanic is too big to fit in the space where the part needs to be installed.

Finally after 3 LONG HOURS, they announce they are putting us on another plane- why they couldn't have done that sooner is a mystery to me, I guess the situation may have been beyond their control but it just left a bad taste in my mouth.
The year before when we flew Airtran, the flight was delayed a little bit b/c the air conditioning was broken.
So, from now on, I'd rather fly w/SWA.
Also, someone said Airtran lets you pick your seats in advance. This perk used to be availiable to everyone, but if you've checked their website you will notice this has changed.
They now have 3 fares:
Coach Lowest Fare: these are the low $79 rates, now you choose seats at check in, NOT online.
Coach Advance Seating: you can pick your seats online w/this fare, but it is more expensive- like $166, I don't think its worth paying an extra $80 for picking your seats in advance.
Business Class: the most expensive, picks seats in advance as well.
Hope that helps! -Stephanie