ContentBox is one of a growing number of products offered by Ortus Solutions. In the CFML developer world, they're, in my opinion, the most active and innovative group. They're actively developing and supporting it, so it's something we felt comfortable using.
The best part is the ease of installation - you can start a new website project, from the commandline, using their Commandbox tool, then serve it from a local CFML server, to begin development; it's literally just two lines.
Beyond that, the interface is clean, modern, and responsive. There are a lot of built-in features for developers ("Geek Settings"), which make site administration and development easier.
Basically, it's a CMS that's built for the modern developer's coding workflow.
It's not as mature as older content management systems (Mura, for CFML, or Wordpress et. al). It's just not as battle tested, and support for edge cases or more complex setups aren't necessarily built in. Parts, like the built-in blog/comments features, are nice for most small to mid-size websites, which would utilize these, but are distractions for websites not wanting to utilize these features. The eco-system around it (their ForgeBox product, primarily), is also still being developed, and not quite mature.
We were looking for an actively developed CFML based content management system. ContentBox meant that, for smaller websites, we didn't need to roll-our-own solution, which is a great time saver.