Pros: Easy to earn miles. No 1st/2nd checked bag fees.
Cons: Difficult to get upgrades/free tickets. Poor service.
I have been flying out of St. Louis, MO for about 20 years. Started on TWA (which was great), have continued with American. The ease of getting upgrades has drastically gone down. It used to be if it were available it would happen automatically. Now you have to earn upgrade certificates.
They have also started cutting back flights, especially larger jet service and direct flights to a lot of locations. More often than not, you will find yourself on a regional jet, with no first class, or connecting through a larger hub. I have seen first class on Northwest regional jets.
The FF miles do not expire, as long as you post some type of activity every year. It try for at least once a month. This can be a flight, hotel stay, car rental, etc.
Difficult to get free flights unless you opt for some level of first class.
Southwest has one of the nicer programs, and everybody is nearly an equal. The flight attendants are considerably nicer, one free alcoholic beverage each way with business class, free peanuts/cheezits, they still have blankets and pillows, and few baggage fees if you pack well.
Continental has a good FF program as well, but limited service from STL.