By the time you start flying ATA, they may have their new 737-800s coming on board. They definitely will be in a brand-new terminal at MDW, which will make your trip smoother.
Having taken many ATA flights, I can tell you that, on average, you can expect:
(1) Plenty of people at the ticket counter and gate to get you checked in, especially as compared with other lower-cost airlines.
(2) Friendly inflight service. Are their F/A's beauty queens? Sometimes not, because many have lots of experience. Do they actually give a damn about whether you have a nice flight? On balance, yes. They have a certain Midwestern hospitality that I like.
(3) You won't get any fancy meal. You will, however, get the full can of soda if you want it, as much coffee and juice as you want without complaint or rolled eyes, and as many of those little biscotti things or peanuts as you want (if you ask nicely). [You'd be amazed at how often on "full-service" carriers, they look at you like you're from outer space if you ask for more of something, or if you pull the f/a call button. ATA's people, in my experience, really are nice.]
(4) The cleanliness of the aircraft is something that I noticed in particular. In a day where the major airlines were castigated recently in the Wall Street Journal over how much crap is left in seat pockets, on the floor, stuck to the tray table, etc. between flights and not picked up by the cleaning crews or the f/a's (many of whom won't pitch in), I was *amazed* at how clean ATA's planes were. On a one-stop flight through Chicago several months ago, I stayed on-board during the mid-trip cleaning, and the cleaning crew (from a private contractor) swept through the plane like a white tornado, with a supervisor who was serious as a heart attack about supervising. It was impressive as hell, and showed that they care about the "little things".
One can have a bad flight any day on any airline. But I think that you are likely to be pleasantly surprised by your experience on ATA.