One of the biggest complaints about the American Express Membership Rewards program is that, while you can transfer points into several different airlines, United and American are not partner options. How can they leave out the two largest airlines?
If you want to redeem Membership Rewards points for United flights, you can transfer points to United partners USAirways, ANA, or Mexicana (and to Delta as well, but Delta points can only be used to redeem United flights domestically).
And you can transfer Membership Rewards points to Continental which, as I explained in my last post, you can then transfer on 1:1 into United. This last option is especially good, because Membership Rewards seems to offer 25% bonuses for transferring points into Continental about once a year — so you can really transfer Membership Rewards points into United at 1:1.25. Nice!
American Airlines is a bit harder, unfortunately. You can, however, take the advice about moving your Membership Rewards points into United miles and then transfer your United points into American via Diners Club. Or you can transfer Continental miles into Hilton HHonors points and then onto American miles.
Both of these options suffer from a real devaluation, though, and should only be done if you really need to top off your American account for a specific award.