I've been with ITT Sheraton Club (now Starwood Preferred Guest) since the very early 90's. Back then, they were about the only one's to pay points for airline staff/travel industry rates, at least on a one for one basis. 10 stays within a year get's Gold level with some associated perks like late check-out (1600 - a real bargain for people like me when all flights back home to Oz from Northern Hemisphere are in the evenings) and more earnings power (3 points per $ spent). I'm more than happy with SPG/Starwood.

I used to be in Mileage Plus as it was relatively easy to get upgrades and the such. However, I now am in another Star partner scheme as you only require 35K miles in a year to earn Star Gold status with all it's perks like lounge access, extra baggage, upgrades etc.
I'm not using it that much for redemptions and the such, but in this day and age when you can find Poz space uber-cheap web type fares and use them rather than play nonrev lotto, then it's nice to have an account to put the miles into.