Our little airport only has Delta or American. We fly Delta. We've had horrible experiences with American and United. If we're flying out of another airport that has Southwest (three airports served by Southwest are within 2 hours of us), I'll see which carrier is the least expensive with the fewest connections, as long as it's not American or United.
The last two times I flew out of our little airport, the American flight to Atlanta was cancelled and they tried to stuff their passengers onto our Delta flight. This also happened this summer at a larger airport an hour away from us. My sister and I were flying to a convention. She booked with American; I booked with Delta. After the third flight change due to flight cancellations, she cancelled her American flight and booked Delta. We met in Atlanta and flew on to our destination together.