Haven't flown Southwest yet (booked for December) so no comment there.
We had a good experience with Airtran on our June trip. We usually fly USAir out of Philly, so I am happy to find this to be a great alternative. The plane was clean and nice, the crew was fine (not spectacular, but fine), check in was easy and smooth, and baggage arrived relatively quick in both airports.
I booked in December for our June flight and the times did change early in the 2005 for our departing flight, but it actually was changed to a more favorable time for us. It didn't move more than 5 minutes from then. Return flight only changed by 5 minutes the whole time. I found it out online, but we did receive two phone calls about the departure flight about 3 weeks before our flight (on home and cell numbers).
I know this may sound trivial, but the other thing I liked about Airtran was they gave us a full can of soda on both flights, not just the little cup. Plus, two bags of pretzels. I know this is trivial, but I do love my soda......