I am still not a member of a frequent flyer program. But I'm most opting for UA Mileage Plus with Star Alliance. I plan on traveling a lot in college, so choosing one program is necessary. I've looked at all of the destinations I want to hit, and Star looks to be the best, even though I would prefer to choose some one world flights over star flights on some routes. The only reason I am leaning more towards star rarther than one world is that the only one world airline out of ROC in American, and to where I usually fly to from ROC (which is RSW), AA has always been way to expensive. Where as with Star, I have the options of both UA, DL, and maybe in a couple of years... USAirways.
I would prefer to fly CX over UA, but its the whole Star One World thing. Who knows, maybe I will opt for One World. I change my mind all of the time. But I think in the long run, Star would be the better option.