Flown Airtran several times with no problems.
Their planes are all newer planes -- either 717s or newer model 737s (I'm pretty sure they retired the last of the MD 80s already). This isn't the airline if you're looking for a lot of amenities or first class service like the legacy carriers offer, but if you want to get from A to B for less $, then Airtran is a good choice.
One complaint I've read on the boards about them is about flight times changing, but that will happen with just about any airline. All airlines change their flight times every few months, so the more time between booking and the flight, the more likely the flight time may change.