As a UA flyer I have to agree with a lot of what was said here but I will say one thing in UA's defense (sort of), the airline is consistently inconsistent...Some days you get absolutely flawless service from dedicated employees, other days you get a group of surly malcontents who just suck.

edit: to add, I just flew UA C on UA901, FRA-SFO on the upper deck of the 747 and it was great. The entire crew was wonderful, the food tasty, and the service was what I expect.