My wife has been running a successful Pixel Art site for a few years now (http://www.myimagination.eu), but she would like to establish some sort of paid membership system, which would give access to extra areas of her site. She found that the most successful sites on the net were using forum CMS as their online sales framework.
I had a look at Drupal, phpBB, Joomla and Invision Power Board. The first three are open source, and the third costs US$150, a little on the pricy side of things.
The key feature I was looking for was paypal-based paid memberships with expiry dates, linked with different user groups/roles. None of the open source CMS I looked at came with this kind of feature built-in. Very few modules are currently written for them, and those that are written are either buggy, difficult to setup, or simply not rich enough in features; they would require significant hacking or much time to configure adequately.
I then took a good look at IPB (Invision Power Board). I must say I was impressed. I tried the demo, read the documentation, examined the various elements included in the license. It all adds up to being well worth its cost. In addition, there is a significant community of developers who create free or commercial solutions for it, and many forums.
Together we decided to pick it up. I will continue to run my site with Drupal, and she will run hers on IPB. We will see, in the long run, which one causes the most headaches :)