Sites running Paid Memberships Pro use it to:
Manage professional organizations with thousands of members.
Get paid for e-learning and digital courseware.
Accept recurring payment for content, podcasts, video, and more.
Create private social communities for unique interest groups.
Develop robust directory or listings sites by topic or region.
Offer product subscriptions or members-only product discounts.
90,000+ active installs.
Recommended by businesses of all types.
I was initially worried about the level of complexity PMPro had, and whether it would do everything I needed it to, but it was simple to set up (with my limited Wordpress knowledge) and easy to customise, plus the support offered when I upgraded my subscription and everything I got access to in the forum was well worth it.