My Leicester Web Design agency has been using the Joomla! CMS (content management system) for at least three years. In that time Joomla! has gone from strength to strength. And so have the 3rd party extensions available. With this platform we've been able to satisfy just about all of our customer's requirements.
One criticism levelled at Joomla!, however, is its inability to accommodate deeply nested content structures. Joomla! organises content along these lines - Home > Section > Category >Pages - just three levels deep. Content Management Systems like Drupal, LiveLink WCM, etc, can handle unlimited nested content levels. The next major release of Joomla! (1.6) will address this 'deficiency'.
However, this 'limitation' of Joomla! is actually one of its strengths! The Holy Grail of web navigation is that no content is more than three clicks away. Joomla!, either by accident or design, reinforces this best practice.
My own starting point is that if your content is more than three levels deep, then you're probably replicating organisational structures rather than putting your customer's requirements first. Therefore, Joomla!'s content 'limitation' is actually a perfect reason to reconsider your information architecture to make your content more accessible and more easily navigable.
So, whatever the size of your organisation and its website, Joomla! is a real candidate to help you manage the website.