As mentioned earlier, this release includes some major refactorings of the internals. Therefore, a lot of MooseX modules had to be updated to take this into account. All of the latest versions of the MooseX modules should now work with both the current stable release, and the latest dev release of Moose, so you can upgrade all of these without any breakages.
The inverse of this is not true, however - many old versions of Moose extensions will break if you upgrade Moose without also upgrading them. When you upgrade to Moose 1.9906 (or Moose 2.0000), the installation process will warn if there are any outdated modules installed on your system that may stop working with the new Moose. Upgrading these modules is important in order to make sure everything continues to work. To make this process easier, Moose now ships with a script named 'moose-outdated', which scans through your system and prints the names of any modules it finds which conflict with the currently installed version of Moose. After upgrading Moose, a simple 'moose-outdated | cpanm' should bring you up to date. This functionality will continue to be provided and kept up to date in future versions of Moose.
Finally, I'd just like to highlight again the issue that Dave brought up in the previous post - make sure you aren't expecting role attributes to pick up the attribute metaclass of the class they're being applied to, as this will no longer happen. Read the previous post for more details about this.
Comments, questions, or issues with the Moose release candidate should be directed to #moose on irc.perl.org, the firstname.lastname@example.org mailing list, or Moose's issue tracker on rt.perl.org.