I first flew UA as one segment (SYD-SFO) of a great deal on a Star Alliance RTW ($US3500 in Business) fare out of JNB at Christmas time 2002. Up until that time I was mainly a One World flyer, but the OW Economy RTW ticket was only a few hundred bucks less than Star Alliance Business, so it was a no brainer. I had to decide which Star Alliance FF program to join, and MP looked like the best deal.
I didn't add to these initial miles until last year when I moved to the US, but now thanks to some travel to LHR for work I'm racking up plenty of MP miles and paying more attention to maximising my return from the MP program.