I've always been treated well with Delta (and previously Northwest - sorry to see them gone). I have had excellent service on Alaskan. Out in the Pacific islands/Asia, where the United staff were all former Continental employees, I their service was also very good. United in the mainland feels like flying with TSA agents, or the bureau of motor vehicles. I can only assume that this indicates a corporate culture of poor customer service, and a general sense that the employee is always right, contradicting centuries of market lessons.
Fortunately for me, even flying to United's hub, Denver, can be done just as cheaply through either of them from where I am.