Joomla 1.6 has just been released. This makes it necessary for me to decide whether to use the new version for new sites or to carry on using Joomla 1.5. Joomla 1.6 is said to be even more powerful and still as user-friendly as before.
The new features did not sound all that exciting after reading up on them, however there are some useful improvements such as nested catagories, a new access control system, easier SEO functionality, better language support and some other stuff that I don't know much about. To find out more about the new features check the Joomla 1.6 site or the What's new in Joomla 1.6 documentation.
That's all very nice but there are still some problems with using Joomla 1.6. The main one being that most of the existing extensions and templates will need to be adapted to work in Joomla 1.6. It will take some time for developers to update their extensions. So if you want to be able to use the majority of existing extensions you need to use Joomla 1.5. Another reason to wait before switching to 1.6 is that there are still bugs in the new system and the upcomming version of Joomla 1.6 which will be released 6 months later is meant to have fixed most of those bugs.