With my Amex Plat and various memberships, I bounce between clubs, regardless of whom I am flying for TATL or TPAC. I tend to use Cathay's lounge at SFO, the Centurion lounge when it's more off-hours, and hardly ever use the United Club.

I didn't know about the First Class UC lounge at SFO; I haven't flown F in a long time on UA and so I doubt I'll get to check it out.