Calling it a CMS is probably incorrect. It is a tool for creating a web site. Unlike Drupal, ocPortal and such it is not a tool to allow you to have the unwashed masses contributing to the site. Where it seems to shine is in allowing a person or small group of people, with little of any clue what HTML, CSS, a pixel, or even what a newline is to put together a web site. This means the look, the presentation and the content.
The user interface allows one, with the right permissions, to edit pages in place. No front end/back end as in Joomla, no separation of a block of content from other things on a page and such. Just an easy to use interface where one can pick what piece of a page they want to edit, edit it and save it.
While this first sounds like a free-for-all, that is prevented by the availabity of page templates and themes. Call it a controlled free-for-all meaning that while one could create a disaster the design at least makes that harder than offering some consistency.
The documentation is good enough. Simple descriptions and videos. There are extensions and additional themes available and very easy to add. My only criticism is that this feature requires you to re-enter some options each time if you goal is, for example, to just see what is available for free.
If you need to build a web presence for a small business, organization or such, Concrete 5 is certainly worth a look.