Good to see so many responses to this thread.
We really like Airtran. We live in the Dayton area, from which Airtran flies daily nonstops to Orlando at great times. (currently 8:00AM from Dayton, return 5:50 PM from Orlando)
There are a couple minor disadvantages. No seat assignments until 24 hours before is a disadvantage for many, but an advantage for those like me who are sitting at the keyboard with fingers twitching exactly 24 hours in advance. I think there is a problem if your Airtran flight is postponed/cancelled; I've heard they don't cooperate well with other airlines in this situatiom, but this has not come up for me.
We've not had problems with schedule changes. Yes there have been some but they were either favorable or by less than 90 minutes.
Some big advantages: Airtran is making money, or at least not losing it, which makes a big difference in employee morale. Most of their planes are 717's, less than 7 years old and with 2-3 seating. This means that even if the plane is full, only 20% of passengers get middle seats. Fares are reasonable if you wait for a sale to buy. (We got $157/RT this year.) And of course for us the fact that it's nonstop from our primary airport is priceless.
When we first flew them in 2003, I bought two Business class upgrades at the counter (we are a family of four). I later discovered that the agent not only comped us the other two upgrades, but also all four upgrades for our return trip! I don't expect that will ever happen again, but it certainly made me a loyal customer.