I upgraded successfully from 4.1 to 4.2 RC2. So far everything seems to work great, and there is a huge improvement in performance and rebuilding speed.
One thing I have noticed that is not working for me is Editing Comments. Sometimes a comment is flagged as Spam. When I click on it to mark it as "Unapproved", nothing happens. It just stays on "Spam". The only workaround I have found is to first "Approve" it, then "Unapprove" it.
This might be a bug.