Three days ago the first release candidate of Joomla 1.6 was published. Given the amount of buzz around this new version of Joomla, and the increasing frequency of questions around whether or not one should use 1.6 now, I wanted to post something about this to share my opinion on the matter.
Joomla 1.6 is in it's first release candidate version, called RC1. That means that this version could be the final version of 1.6, unless there are any major issues discovered. At the time of this writing, several bugs have already been identified that are known as blockers to a stable release. So, there will likely be several RC versions before a stable version is released. (For comparison, I believe there were 4 RC releases for 1.5).
In addition, there will be security improvements to Joomla 1.6 that will likely be added before it is reaches a stable version.
1.6 has a few very nice additions to the functionality of Joomla, but I strongly recommend not using it until it is released as a stable version. You'll know when it's reached a stable release because the "RC1" or "RC2" or whatever RC version is the last, will disappear from the release name, and will be replaced by something like "stable."
Now, even when 1.6 is stable, you may not have a reason to migrate your 1.5 site to 1.6, at least for quite a while.
The biggest additions to 1.6 are the Access Control List, or ACL, and infinitely nest-able categories. If you really need nested categories, or if you need the benefits that the new ACL system provides, then you should consider using 1.6, after it's stable.
However, there's another important aspect to keep in mind: there is currently only one extension, out of thousands, that is compatible with Joomla 1.6: Akeeba Backup.
Unlike the transition from Joomla 1.0 to 1.5, the transition from 1.5 to 1.6 will not provide extension developers with the option to run their 1.5 extensions in a legacy mode within 1.6. Their extensions will have to be rewritten for 1.6, which is going to take some time, and a lot of work.
It will likely be many months after 1.6 is released in it's stable version before you'll see many of the top extensions released in a 1.6 compatible version.
So if your site relies on any extensions for it's functionality, you'll need to either find other ways to recreate that same functionality, or wait until your extensions are released in a 1.6 compatible version.
Joomla 1.6 is scheduled to be released in it's stable version in the second week of January, 2011.
Whenever the stable version is released, it could be 2, 3, or 4 months (or more) before you see a large number of extensions released in 1.6 compatible versions.
Aside from extensions being released in 1.6 versions, templates will also need to be 1.6 compatible. While there is less work involved in translating a 1.5 template into a 1.6 compatible version (less work than re-writing an extension for 1.6), there will still be work involved, and not all templates from a given company will likely be ported into 1.6 versions.
So, in my opinion, I would highly recommend waiting for a while before you start using 1.6 for a new site, or before migrating your 1.5 site to 1.6 (the migration tool is not yet complete either, as far as I know.)
If you need ACL, nested categories, and the handful of other new additions to 1.6, if you have the extensions you need for a 1.6 site, and if you have a 1.6 compatible template, then go for it (as long as 1.6 is in it's stable version.)
Otherwise, I would wait until more extensions and templates are released for 1.6 before starting to use it.
After 1.6 is released in it's stable version, we will start creating new versions of our video lessons for Joomlashack University, starting with the beginner course. This will take a large amount of work as it will require re-shooting and editing all of the beginner course videos using Joomla 1.6 (and adding new videos to cover the new aspects of 1.6), so it's not going to happen overnight. But there will eventually be a complete beginner course for 1.6.
Until you're ready to move to 1.6, which should be a while, keep using 1.5. It will be a long time before 1.5 reaches it's end of life.
And, keep in mind that there's been discussions in the Joomla developer team about releasing Joomla 1.7 within a year after 1.6 is released (I've heard that 1.7 is scheduled to come out 6 months after 1.6 stable is released, and 1.8 is scheduled to come out 6 months after 1.7 stable arrives.) So we may see a lot of Joomla site developers/designers, and maybe some extension providers, skipping 1.6 altogether and waiting for 1.7, which is set to have significant changes made to the user interface, among other new additions.
If you're concerned about another big migration of your sites from 1.6 to 1.7, the upgrade to 1.7 is not supposed to be as major as the upgrade from 1.5 to 1.6.
As far as I know, Joomla 1.6 is also not intended to be a long term supported release, like 1.5 is. This could mean that when 1.7 comes out, the Joomla development team could fix bugs on 1.6 for 6 months after 1.7 comes out, and then any bug fixes would be put into 1.7. Or, it could mean that support for 1.6 bug fixes will end when 1.7 is released in it's stable version.
I've also heard that the Joomla developer team will support Joomla 1.5 for another one and a half years or so from now, in parallel with 1.6, 1.7, and 1.8. (If anyone on the core dev team would like me to correct any of this info on the timing of releases and support schedules for versions, please contact me and let me know.)
I hope this has helped address any questions you may have had about the immanent release of Joomla 1.6.