Well, we were hoping to move from beta 5 to release candidates, but it's just not going to happen. We're more committed to a solid core than we are to hopping on through development phases as anyone who roughed it through the alpha phase can attest. There were too many minor bugfixes, particularly with a couple queries and admin pages, to consider moving on into release candidate territory. We'll let the beta 6 sit for a while to see what else pops up and maybe get a release candidate out before or during the Boston Drupalcon.
There aren't any major changes to report that would prevent you from updating. Mostly this is a release to propagate the query and uc_product_node_info() fixes that keep popping up in the forums. There have been a few other minor fixes and tweaks added to the code as well.
The latest release is available on our project page - http://drupal.org/project/ubercart
Refer to this FAQ page on updating your site if you're unsure how to do it or need a simple updating checklist.
You can check out our Bazaar log for commits since 2-8-08 for changes since the beta 5 release.
If any issues pop up, don't keep them to yourself. Let us know so we can get out of beta territory once and for all. (Well, at least until 2.0!)
EDIT: There is a non-critical error popping up when you run update.php for an incorrect database query. It breaks nothing, will cause no problems in the future, and has been reported and fixed.