@davidofadelaide clued me in to a great review of concrete5 in Linux Format Magazine. In issue 161 (September 2012) they put us up against Cotonti, Drupal, Joomla and Exponent CMS. The review itself is fairly lengthy and judges based on a number of different criteria, including extensibility, third party add-ons, theming, ease of use, ease of installation, user interface and documentation.
Some choice quotes:
concrete5 has the simplest installation of all, and is finished before you can say Eyjafjallajökull
concrete5 allows for in-line content editing...You can also make design changes for each element by clicking on the element and choosing Design. This is done so easily and efficiently that we wonder why other CMSs don't offer the same functionality.
Whether it's adding or modifying content, or configuring the site, the concrete5 experience is an absolute delight.
I won't post any more of the review, but I'd encourage you to check it out (and not just because I like the end result, which ranks us ahead of the rest.) Thanks very much, Linux Format magazine. If you'd like to download the full article, you can do so here. (I purchased it via Newsstand in my iPhone, and plan on reading the rest of the articles once my iPad is again in my hands.)