I have quite a bit of experience using web-based Content Management Systems. I have used and developed extensively for Mambo, Drupal, and most recently Wordpress. I have tried out a handful of others, but I cannot say I have an in-depth view of them. As far as popularity goes Mambo/Joomla, Drupal and Wordpress probably account for a significant number of CMS/blogging platforms on the net (at present). So, take this from my user-developer experience, and usually from the perspective of something I call developer-usability. Developer usability is an important factor in choosing a tool to manage your content online, and hopefully developer usability and user usability are high goals on the platform's agenda, In fact, usability is a not-for-negotiation goal/feature.