I was recently checking out the cool Google Trends. I did a simple search to see the difference in the usage of the 4 major players in the CMS world (Joomla!, Mambo, Plone, and Drupal). If you look at the trend graph here you can see the split between Mambo and Joomla!. Drupal and Plone are nowhere close.
Now this doesn’t say that Joomla! is better than any of the others, its just showing that more people search Google for Joomla!. It’s the popular one. But the trend with open source projects is that the more people like the project, the better it is.