This release of cTools includes most of the RTBC issues from the 7.x release. There is a mix of new features and bug fixes.
Because of the amount of RTBC issues, we're doing a RC1 release first to make sure there are no regressions. While all the patches passed tests and a smoke test, there could be issues and we want to make sure everything is stable before rolling a 1.6 release.
After a 2 to 3 week test, we will roll a full 1.6 release. If you want to help out, ping japerry in irc.freenode.net #drupal-panels or #drupal-contribute!