I have had issues with all of the major US airlines, but fly mostly SW and AA. Hopefully this won't jinx anything but overall my best and most consistent experiences have been on AA. I'm posting below my rant on my recent United experience that led to my "never again" vow.
"Apologies for the following rant but I want to warn everyone on my lists about United Airlines. Here is my experience about getting to Phoenix from Cleveland Monday to Tuesday last month
"I was stuck in Houston International from Monday evening to Tuesday afternoon due to the incompetence, uncaring and chaotic mismanagement of United Airlines. I have already commented to them about their monumental mismanagement of this crisis. I arrived in HOU from Cleveland at around 7pm( I initially left Pittsburgh at 9am to go to Cleveland and the R&R HOF). Staying at the gate, we were not told the flight was cancelled until around 2am. Indeed about every 20 - 30 minutes after the initial departure time we were given a new departure time. Amid nascent fist fights and the arrival of 3 Houston police, we were finally told around 2am that Chicago in their brilliance had decided to cancel the flight. That triggered the "what about my luggage" scramble that sent a plane load of people to baggage where we were given mis-information that it would be available at 6am. No it wasn't then about 6:45 the beleaguered baggage attendant announced that she wasn't accepting any "retrieve luggage". requests and even when they were accepted it would take 4-6 hours to retrieve the luggage. Out of desperation ( the flight was originally scheduled to Phoenix), I saw a staff member that appeared knowledgeable and asked her if "since my bag was going to Phoenix, might it have been sent on ahead." Yes she answered as I immediately booked a flight on Southwest for an additional $400+ and arrived in Phoenix at 6pm and, mirabile dictu, my luggage was waiting for me at Terminal 3. I. Will. Never. Fly. United. Again!"