For a very long time, the idea of the x.5 series of a Joomla release is the stable and official version that you should be using and for a long time people would have been waiting for this release before upgrading, migrating or developing for the platform.
This is no longer the case and had not been since ~ November 2013.
You no longer have to use and wait for the x.5 release of Joomla. You just use the latest version of Joomla that you can download.
No longer are there two different or multiple versions, e.g 2.5.x and 3.1.x, of Joomla that you can download but only the one latest version.
Support for a Release
Previously the x.5 series release was the long term support and would have been supported by the project for a long period of time. This created a period of stability meaning that major upgrades or updates were not needed. Now that the rolling updates are being embraced, each version that is released will be supported for two years from its release date. This means that as long as you update your version of Joomla to the latest patches, you will be safe and secure and taking advantage of all the great new features in Joomla.
So what are you waiting for.
It's time that you're using the latest version of Joomla, Joomla 3. It's safe, secure and feature packed with SEO features, new publishing features and much much more.
What's New in Joomla 3
Features in Joomla 3.0
Features in Joomla 3.2
Features in Joomla 3.3