I was really impressed with the overall usability of the RadiantCMS software, but most of the features beyond having simple static pages were provided by 3rd-party plugins... many of which were out of date with the core software by the time I tried them, and failed to work in mysterious-to-newcomer ways.
I like Radiant's simplicity. A PHP port of it is FrogCMS. I don't see myself using Wordpress as a lightweight CMS again, Radiant/Frog are easier to setup and maintain, and the code is well organized so they're hackable too. Every time I need to dive into the PHP of Wordpress I get frustrated with it's spaghetti-ish ways.
I've been working on a large-ish Radiant site and grabbed a few plugins. So far all have worked as expected, except for one for page-level user access control.
The problem with that one was that the author did not account for having thousands of users and a hundred or so pages. I added pagination for the user list, and created my own plugin to handle page groups so I could assign page permissions en masse rather than one by one.
It's way nicer than having to poke into any PHP I've dealt with. Biggest complaint so far is that I often cannot find docs clear and complete enough to help me understand something before I go and hack my own solution.
But the upside is that when I later realize I should have gone another way it's easy to refactor, and using rspec and selenium tells when when things break.
