Concrete5 is great for getting prototype websites from static html to dynamic content full CMS systems in a short period of time. The learning curve for someone who has no experience with creating themes for CMS' to using Concrete5 is almost negligible.
Concrete5 is the least contrived CMS I've ever run across. It's easily the quickest to theme and get up and running whereas Wordpress or Drupal could take months to get a Theme made Concrete5 can get one in a week. There's no reason to re-invent the wheel with loops and the such like WordPress. Pop in a bit of PHP code into a basic html page and it becomes a dynamic text box.
I have recommended it to several people. While I would still suggest something custom engineered for a lot of complex sites, there is no reason why 80% of sites built ontop of WordPress shouldn't be made with Concrete5.
I've been doing less and less web work recently but if I were to go back I would most likely use it again. As I've said before Concrete5 is very good at quickly creating a web presence. I'd definitely like to go back and see what new features it has.