Priority Pass lounges are either actual Priority Pass properties (more common overseas) or participating airlines. I've used my Priority Pass Select in everything from TACA to KLM. I specified AA as my carrier of choice with AMEX, so I only need an AA boarding pass, my Plat AMEX and a gov't ID to get in to Admiral's lounges. Priority Pass participating lounges usually have a sign up, but there are a few that do not honor Priority Pass SELECT (the one you get with the AMEX Plat, I guess there is a level above SELECT, but with all the properties you can use, it isn't a problem). I've had this happen maybe once in the last 4 years. Priority Pass has a nice smartphone app that will tell you the closest participating lounge.
Lounges are lounges and I think they really depend on the specific location more than the parent company/airline. The Admiral's Lounge in Philly circa 2008 was a dump, reminded me of a trailer park. The flagship one in MIA is pretty nice (showers, food to order, solid well drinks and beers), and the satellite one in the same airport is also decent. The one in Santiago, Chile is phenomenal, with a serious spread of free (good) food, full serve-yourself bar, smoking lounge, blazing fast internet (good for streaming MLB.com or NFLTV on a long layover). But in the end even the worst lounge is better than the terminal.
The only "Priority Pass" branded lounge I use is in San Salvador, and it does the trick, 2 free premium cocktails (really 4 the way they pour), free snacks, bigger food to order (good, cheap), quiet, free internet, couches/workstations/bigger tables, fast and friendly staff, private bathrooms, and the rare and treasured power outlet.