In United's defense, while I have not had the opportunity to fly them myself, I have placed several dear family members in their charge, both with difficult issues (envision: teenagers, bizarre packing habits, attitude), and all handled with consummate professionalism and courtesy. Maybe just the DFW station, but in my experience, a company's employees are either interested in providing service, or they are not.
U's folks are (from what I can tell) enduring a lot of crap as consequence of the combination of poor management (at the top), obstinate labor issues (IAM, thanky very much) and unavoidable circumstance (9/11, anyone?).
Much of what has been written can apply equally to any large, "legacy" carrier, from what I have seen.
Try this some time: Go to the airport, or board a flight, and greet any particular employee of the airline (could be U, or any other carrier). Tell them, "You're all a-holes who don't care, because I had a bad experience last time I flew."
Never happen. Not with the crew I find here, because (by and large) you're all part of the "good egg" aviation community. And we (most us) know that, on any given day, most people want to do a good job, and enjoy giving good service, and sometimes it just doesn't work out.
So cut the folks some slack, allow them a bad day every now and again. And, by all means, do as I do when you see exceptional service (like that U agent who went out of his way to find a box to package my nephew's random and obviously not-allowed-in-carry-on-luggage crap, so it could be checked instead of left behind, or the CO agents who went out of their way to make sure my sister's bags were checked through all the way to Adelaide, South Australia on her no-kidding three-airline-five-segment-flight, rather than telling her she could just claim bags at LA and start over), and send a letter to the airline's president telling them what great extra effort these particular people went to to take care of customers, and make the airline look good.
Just my idea.