If you studied already the several release candidates (RC), you may know, that Joomla 1.5 is more than a simple updgrade. The extension installer has been simplified an enables to install extension packages and additional languages now in a single menu. An integrated editor is provided now and to create menu structures the flexibility has been improved a lot. Furthermore it's now possible to generate search friendly url's out of the box using the main configuration of  Joomla! 1.5. 

Joomla was created out of the CMS Mambo. The release 1.5 leaves a lot of old structures behind itself and shows up as a big milestone in the history of Joomla 1.5. The core developers putted a lot of energy in improvements like usability, clean code and better handling. The well known joomla-feeling is still the same although. Joomla 1.5 supports still PHP 4.3 as well as PHP 5.0 and higher.

Who likes to migrate his existing Joomla-website up to Joomla 1.5 has nonetheless not an easy step to take. Not all the extensions are running with Joomla 1.5 and the choice of Joomla-1.5 native extensions is still not overwhelming. But for the task of migratio an assistent is trying to make this step easier.Furthermore you can run old extensions in a backward compability mode, what is working out fine, but not for all extensions. Long testing is necessary anyway after migration of a bigger site.