I use Mambo for many of my sites. Clients really like being able to modify content one the front for the site without html/ftp knowledge.

Mambo seems to have more module and component addons than the other CMS'. I like being able to install them with a few clicks.

Mambo isn't perfect for everyone. SEO issues and the recent split into Joomla would give me pause if I were considering learning the system from the begining.