I completely agree about US Air. Their service is better than any other airline that I have flown. I hope they don't get bought by UA.

As for CO/NW, I was tempted to conclude that membership in both programs would be redundant, but for the partners! Often America West and Frontier have the best deals for transcontinental travel, but who wants to be a member of their FF programs? Since CO (and only CO) partners with them, it does make sense to belong to the CO program.

On the other hand, belonging to the NW program makes sense because you can accumulate so many bonus miles with them...