I recently bought into Priority Pass (after looking here for comments) and was VERY impressed. It cost me £159 for one year and allows 10 visits to around 450 lounges worldwide. After that you pay £15 a time and can sign guests in. Other versions include unlimited visits £250ish and also a no visit pay as you go £60. I figure the cost of 10 years membership equals roughly the cost of one transatlantic business flight.
The lounges vary in quality, I wasnt too taken with Servisair in T3, but I liked the one in Dulles which is pretty good. A lot of decent airlines participate and there are a lot of lounges in less salubrious places where facilities are scarce. This makes it worthwhile in my opinion.
Amazingly the Priority Pass will apparently give members access to the lounge in Lisbon where I too fell foul of BA's ticketing.
All in all a good deal, well worth the money if you fly regularly but dont fly business. It makes the airport experience bearable - also there are quite a few arrivals lounges listed as well. Definitely look into it. (No I don't work for them, but am very pleased so far with the level of service I've received).