By default, new users are blocked out of the registration process because the plugin blocks the registration page if it has parameters tacked onto the URL. I got around this by adding it as a regular expression to my public URL group.
Once the user is allowed to register, the Paypal express button code is broken and some of it displays to the user, as shown in the attached pic.
The most major issue for me, which I seriously hope can be resolved, is that protection for subdirectory blogs does not seem to work at all. When I enable the Membership plugin, certain member blogs are displayed fully to the public, while others show a page not found error!