My PP subscription is included in my Amex Platinum Card subscription - free entrance for me, £15 charge for guests.
Advantages:
1. gives extra choice of lounges when travelling premium - especially useful when some lounges such as BA get very full;
2. gives access to lounges when travelling economy;
3. (for UK cardholders at least) gives access to lounges of Virgin, TWA, British Midland and Eurostar rail when using those services, no matter what class of travel.
Disadvantages:
1. variable quality - some excellent, others less so;
2. no PP lounges in Australia!