is nice and has a lot of extensions.
It is also quite a handful for someone who wants to begin using it for many different clients, unless you get into making your own templates ... not for the faint of heart. It runs best on a dedicated server, or at least one where the admin has the ability to modify permissions and ownership of files and directories. You'll also need to have at least a little bit of familiarity with MySQL to get the most out of it.
Once you understand how things are linked together (menus, content, modules and components), Joomla can be a very useful CMS system. It blows many other systems out of the water with regard to robustness and security, and the extensions can be a lot of fun. There are more simple CMS systems out there, and more complex ones, but I give Joomla a thumbs up for anyone who wants to dive in and learn about it.