The dramatic variance in service quality on UA flights has always astonished me but I usually try to understand that a specific FA might be having a bad day. A couple of years ago, I was on a five hour flight in F from PDX to IAD and the FA hid behind the curtain, by herself, for all but a half hour of the journey; just about enough time was allocated for a very quick meal service before retreating back to her galley of solitude. I was not in a particularly ornery mood and I thanked her for her service as I deplaned, as is my custom. This was not a sarcastic thanks but a sincere, if not overly heartfelt, one.

I think that I caught her off guard because she knew the service had been lousy and, I guess, could not imagine why anyone would dream of thanking her. I don't remember all of the details at this late date, but she told me that she didn't feel well, or there were some personal problems, or some combination thereof. After we chatted for a few minutes, she felt better and so did I.

On the other hand...UA boasts entire routes in which I always seem to enjoy consistently inferior service. As has already been pointed out, UA's trans-Pacific service is typically quite mediocre. But my personal nomination for the route in which I nearly always endured subpar and indifferent service are my flights from IAD to MIA. Over the last six years or so, I have flown this route over two dozen times, and except for a couple of occasions, FA's do not bother with a pre-departure drink, taking my jacket, returning my jacket or even smiling. After spending an entire five minutes feeding and watering the passengers, the FAs hide behind tightly drawn curtains for the entire duration of the trip. My last such journey in January prompted me to write my little love poem to UA entitled "50 Ways to Lose Your Flyer:"
http://www.flyertalk.com/forum/Forum50/HTML/004576.html

The scary part is that I am scheduled to do this trip again on April 17. One curious observation is that flights from MIA back to IAD do not seem to be nearly as bad.