I have been a Premier Executive (50k+ miles a year) on United for the past three and a half years, and was a Premier (25k+ miles a year) for a year and a half prior. United's frequent flyer program is terrific: in five years of membership, I've accumulated 500,000 miles (used quite a few, of course, but steadily rebuilding).
I must say that as much as it's a rewards program, it's also a loyalty program: even on short flights, I fly United 80% of the time because I genuinely like flying with them. Star Alliance has proved to be a boon for United, at least from my perspective, because it allows me to fly
BMI, US Airways, Lufthansa, Singapore, SAS, and Thai (plus the others, but haven't flown them yet) and still earn miles. Or, better yet, redeem miles for premium cabin tickets on
SQ!
I've also had a fair amount of experience flying Northwest, and a lot of flying on American. Before I changed my loyalties to United, I was an
AA-exclusive frequent flyer for 12 years, except when the destination required an alternate carrier such as PIA or Emirates.
"The education of a man is never completed until he dies." - Gen. R.E. Lee