Joomla! is different from the normal models for content management software. For a start, it's not complicated. Joomla! has been developed for everybody, and anybody can develop it further. It is designed to work (primarily) with other Open Source, free, software such as PHP, MySQL, and Apache.
It is easy to install and administer, and is reliable.
Joomla! doesn't even require the user or administrator of the system to know HTML to operate it once it's up and running.
To get the perfect Web site with all the functionality that you require for your particular application may take additional time and effort, but with the Joomla! Community support that is available and the many Third Party Developers actively creating and releasing new Extensions for the 1.5 platform on an almost daily basis, there is likely to be something out there to meet your needs. Or you could develop your own Extensions and make these available to the rest of the community.