Make sure to pay attention to the particular clubs included for the airports you fly through most often when doing so, because for some airports, the lounges that Priority Pass gets you into may be in entirely different terminals than the one housing UA if the RCC isn't included for that airport (which is the case for IAD and some of the other UA-heavy airports). I'd not want to have to transfer terminals and potentially re-clear security twice just to use the airport lounge.