I fly on AA out of STL on a regular basis. As an airline, they're not significantly better or worse than any of the other legacy carriers. The ST airport is fairly easy to get around in this day and age; although it's a bit (maybe more than a bit) run down. You can check in and check luggage curbside (for a fee of $2 per bag), or there are plenty of self-service check-in kiosks inside or you can use the regular human-staffed check in lines. Unlike some airports, once you check your bags AA takes them for you and you don't have to schlepp them over to a security screening area.
The security checkpoints aren't the most efficient and at the main checkpoint for the C concourse, where your gate will be, they have maybe three lines open if you're lucky. As a result, you can get stuck with a wait there. (Hint: If the line is really long, check out the smaller security checkpoint at the B concourse, which sometimes has virtually no line at all. It may not be open when you'll be there, however. If it is and there's no line, you can use it instead, then take a right and head over to the C concourse.)
Once past security there's a food court with a Burger King and a Cinnabon, a Wolfgang Puck Express, a Chili's Too, a couple of Starbucks and some random gift/sundry shops.
Oh, AA is a participating airline in the RAC, so if you are staying at a Disney resort you can check your bags and get your boarding passes right in the lobby on your way home. This will make your airport experience much more efficient.
