My first flight EVER was in June 1976, and it was on UA. Having graduated from college in the Boston area, I took a job for a large company who decided to station me in a midwest plant, so I went to my new job location without ever having seen it before. I flew UA BOS to ORD, connecting on what was then North Central to my final destination.
I joined MP shortly after UA introduced it, but then again I joined every domestic FF program, as sometimes I didn't have a choice of preferred carriers. Through 2000 I had only flown about 30K BIS total on UA. I moved to the L.A. area in 2000, and it was obvious my previous loyalty to US wasn't going to work there, particularly as my new job requires travel around the world. UA was the obvious choice. I was less than enthralled with the idea at the beginning, but I've grown to become quite loyal and really enjoy UA's unique features like E+ and Ch. 9. I'm now over 350K BIS miles total, and a big UA fan.