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I've used XOOPS, Joomla and Drupal. Drupal is the smallest in size but most complicated in details. Joomla has pretty GUI (IMHO, I think pretty GUI is important for web design and for attracting eyeballs. Hope XOOPS gets prettier and prettier. Nowadays beautiful GUI is crucial to commercial websites although the techniques are equally important) but its security is not as good as XOOPS and Drupal.
I tried to develop themes for Joomla but it seems more difficult than XOOPS (even compared to template-style themes of XOOPS). XOOPS' structure seems to be easier to be understood (maybe owing to its long history). In addition, the size of Joomla is getting larger. In fact, XOOPS has this trend too. I do hope the XOOPS developers should be concerned about not getting their file too large. Drupal is much smaller and easy to install. However, much works must be done to modification, adding modules, modifying themes, except you just setup your blog.
For community website, XOOPS is still the best choice. But for commercial website, much works must be done on the GUI, themes and easiness on installation (especially without the moving of XOOPS_data and XOOPS_lib by user maually). The incorporation of ajax and other Web 2.0 (or even Web 3.0) techniques are also important.
In China, some XOOPS 3.0 test sites are available and the new features are amazing. Thanks for phppp and all XOOPS developers. XOOPS is a masterpiece indeed.