They are a solid LCC with a large and growing east coast presence and an aggressive plan for growth. They are among the four airlines (the others being JetBlue, American, and Delta) that have expressed interest in pieces of US Airways. They have a fleet of 717s that fly out of ATL primarily. 737NGs are on order for delivery starting fairly soon. Check out airtran.com. Stock symbol is AAI.
I've flown them a couple times ... safe, comfortable, and inexpensive. Unlike many other LCCs, they do have a business class cabin on board, which is nice, and they've also got small jet feed (courtesy of Air Wisconsin) to the ATL hub.
I'll be surprised if you don't begin to hear more about them out west ... that's been a major target for expansion for them. They've announced LAX and SFO service, and are flying them now with A320s contracted through Ryan International.
FL270