Just got back from Aruba. First time flying AirTran.
You have to pay extra if you want to book a reserved seat at the time of buying your ticket. Depending on location, it will cost from $6 to $12 (on my itinerary anyway) in coach. Business class is more. You can take pot luck at the time of check in & they will assign you a seat at no charge.
If any part of your itinerary includes a segment on Southwest, no bag fees. Hurray!! That saved me around $70 right there.
I don't know if this will still be accurate when Southwest completely takes over AirTran, but that's the way of it right now.
I was pleased with my flights (MCI-ATL-AUA and AUA-BWI-MCI) and will fly AirTran again. I used Southwest points, and converted them to AirTran segments. Happy girl.