Does the plugin accept memberships / originate passwords / resend lost passwords / manage members and all the functions of a membership backend or is it just a payment gateway?
All the functionalities of WP are still managed by WordPress, but s2Member adds functionalities, like protecting content for members according to their level, membership, custom capability, selling access to individual pages and posts or group of them, and much more.
Since the basic version (not Pro) is free, you might want to have a go at it, install it (even better if you have a test site), check all the functions, even read in this forum to see what people are doing (even if they tend to post when they are stuck, you might get an idea of the power and flexibility).
Good luck