I am still using Joomla! 1.5.3 on some sites.
What about the Joomla! 1.5.5 and previous 1.5.4?
Let us just focus on the new release.
The Joomla! community is pleased to announce the immediate availability of Joomla! 1.5.5 [Mamni]. This is a quick turnaround release to address the Duplicate Title error from 1.5.4 . This release also contains important SEF URL improvements and fixes for com_content in addition to a number of bug fixes and improvements. It has been nearly three weeks since Joomla! 1.5.4 was released on July 8, 2008. The Development Working Group ‘s goal is to continue to provide regular, frequent updates to the Joomla! community.
Here is the official announcement.
For me, I don’t want to upgrade it manually. I am waiting for the Dreamhost one-click installation.