<font face="Verdana, Arial, Helvetica, sans-serif" size="2">3) AS - many people can't use AS miles because their east coast coverage is limited, but for those of us that have business/pleasure in the west, east, and northwest, the 20K award is a great value.</font>
I have always found AS miles to be the easiest miles to use because they are partners with half the airlines that fly in and out of the U.S.
Also, haven't had a problem using UA miles, even during holidays and at the last minute.
Our goal is to build up a large quantity of both UA and AS miles before we retire so we can fly pretty much any airline we want once we finally have the time.
Actually the best value we ever got from points/miles (thanks to
Ozstamps) was trading in 120,000 Starwood points for 300,000 Qantas miles, which we in turn traded for two Concorde tickets.

We still have 50,000 Qantas miles left in our account.