Prestige only provides access to an Admirals Club when you are flying AA, and (unofficially) US. It does not provide access when flying UA or DL. Have you found otherwise?
No, I have not. I only AC when flying AA. I'm just saying I wouldn't pay for AC membership since I would not use it enough, and paying $50 a day also adds up. Using a co-branded credit card that includes the benefit of accessing an airline's lounges works better for me, as long as the other benefits of a given card add enough value to justify the AF. So far that has been the case with Prestige.