I love this default enrollment plug-in. By a long shot it is the easiest way to turn a Moodle into a revenue generating site (selling course enrollments and/or certificates). Visit http://docs.moodle.org/en/Paypal for more information.
The Paypal module is easy to setup in pre-2.0 installations (I have not tested it in Moodle 2.0). In general it requires very little administration or setup (though I do recommend using the email-based self-registration authentication to allow users to easily register and then pay for courses.
Are you using the Paypal enrollment plug-in? If you are…what subject/level courses are you selling?