Very interesting question, which has a very complex answer, IMO. It really comes down to each person's perception of value. For example, if you fly UA regularly, and want to upgrade, you will obviously get the best deal with UA miles, but that kinda goes without saying.

That being said, I find the overall cost with UA and US to be excellent for awards, especially compared to some of the partners. They tend to charge less miles for the same route, based on what I have seen, but maybe someone can correct me there. Again, I can only really speak from UA, but I find great availability overall on the Star carriers, so it is rewarding in that sense.

So I would say that UA provides an excellent value assuming you want to redeem for awards as opposed to upgrade on a certain airline.