The Joomla team put this upgrade out without saying to much about it. All the users of Joomla who stay on top of upgrades and act quickly found out the hard way that there were serious issues with anything considered to be a bridge. Personally I would have never call Community Builder a bridge but turns out that is what they consider it to be. The login module for Community Builder replaces your Joomla login module. 

After completing the new upgrade installation many Joomla site administrators found themselves locked out of there administration area. The same problem happened to their sites registered users. This was about the worst nightmare an admin can go through! What caused the problem was a major change in how Joomla handles passwords.

If you decide to do the upgrade to 1.0.13 make sure you remove Community Builder first, then upgrade by over-riding the files with the new ones. Then re-install Community Builder.

You should not have to worry about losing any Community Builder data, like your registered users. You are only un-installing CB, not emptying the database tables. Having completed a half dozen upgrades so far, they all have worked and the new Community Builder installed fine, no registered users were harmed during the procedure.

Are you trying to decide about upgrading your Joomla 1.0.12 to 1.0.13? Your not alone, thousands are just starting to look at the issues that can come up. I would suggest reading what a Quality and Testing Workgroup member of Joomla has to say at Virtuemart's site .