I started using Joomla when it was on Version 1.0. Looking back it has come on a long way. It transitions to Joomla 1.5 which was a major upgrade and has now transitioned to Joomla 3.0 which was released late 2014.
Joomla 1.0 was good for it's time but slow and clunky, Joomla 1.5 improved a lot, Joomla 2.5 a massive improvement and 3.0 is a pleasure to work with.
This was also a major upgrade with a rewriting of the system from the ground upwards. Unfortunately, Joomla has stopped supporting Joomla 1.5 and 2,5 and it will leave the site vulnerable to security risks in the future and they stopped supporting 1.0 a long time ago.
Now is a good time to upgrade your site to the brand new version and the good news is that any furture upgrades can be done with the click of a button.
Upgrades are recommended immediately if you run shopping software or any other storage of personal data. Security fixes will no longer be issued for joomla 1.5 which will leave your site vulnerable to hacking.