Joomla is one of the CMSs with which I most often work. While I wrote "What's New for PHP on Windows: Developers Get More Done with Joomla!", most of my own commercial development has been Linux-hosted. Part of the point, of course, is that such low-level details as operating system are largely irrelevant: Joomla, like other good CMSs, allows contributors to concentrate on content, not machine matters. Joomla (or, more officially, "Joomla!") is particularly strong outside the US as, for example, Rikki Endsley reports.
My own involvement with Joomla is similar to what I've done with other CMSs, in that it's concentrated far more on reporting and integration with other organizational assets, rather than standard configuration and theming. A crucial resource for me, therefore, is Torkil Johnsen's schema (which I like to view as SVG, of course, because of my affinity for that format). Also, therefore, I end up doing a lot of PHP coding.
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Inevitable topic: how does Joomla compare to Drupal and WordPress? Julie Bort nicely provides a few facts.