Joomla is a very unique Open Source project. Joomla never had a benevolent dictator or a corporate backer. There is no office building and Joomla has no paid staff. Still, Joomla has grown to be one of the most popular CMSs in the world, thanks to volunteers!
Joomla is very flexible and has many possibilities. A disadvantage is that working with Joomla can seem complex because of all of the setting and navigation choices. For a Joomla specialist this is usually not an issue, but for many end users it can be a stumbling block. Adding new articles via the Joomla frontend is often one of the first things end users face. This should be a simple operation, but many experience this as very complex.
Joomla 1.5 is not supported by the Joomla project for quite some time now. You still can continue to use Joomla 1.5, but (security) updates will no longer be released. Furthermore, both Joomla 2 and Joomla 3 have many new features and improvements. Perhaps you recognize it: once you use Joomla 2 or Joomla 3, managing a Joomla 1.5 website really feels old-fashioned and you notice how greatly Joomla actually improved.