I am a United Airlines Mileage plus member. (Also valid for all star alliance partners).
No problems at all I always got the mileage that I deserved. I joined their program because I am a regular visitor to the USA.
Right now I have enough mileage for a one way upgrade to business class on my next UA flight to the USA......I am already looking forward to that experience.
Yes, I am one of those ordinary people that can't afford to fly Business class unless it's a business trip....