If you need some form of membership or subscription facility on your site and/or a way of tracking and rewarding referrals, then you shouldn't hesitate too long before downloading these modules.
Paying for a Drupal module might give you pause when so much is free, but it is a very modest sum really, when you consider the depth of functionality, and the amount of time that you are saving by not having to do a lot of programming yourself. There should be plenty of room for both approaches!
As a relative newcomer to Drupal, who is managing to write his own modules, but often having to spend a lot of time working things out, I have found the way these modules are written to be extremely clear, and very easy and instructive to integrate with. I have actually only had to do that because of some very specific requirements. For most purposes I am sure they would work immediately you have installed them, as there are many configuration options.
Support is first rate. It is unlikely that you will need it in fact, but I had a very strange problem (which embarassingly turned out to be of my own making). Leighton was onto it straight away, and investigated it fully until he had tracked it down. Many in his place would have dropped it at the point of establishing that it was not his code at that was at fault.
The modules are being continually enhanced and I am looking forward to benefitting further from the new facilities as they arrive.