tl;dr 42.2 Beta 1 is on the mirrors, get it from http://download.opensuse.org/distribution/leap/42.2/iso/
Even though it's called Beta 1 the base system is mostly in sync with SLE12 RC2. Therefore it's pretty safe to use on most systems already. So please put this under thorough test and file good bug reports¹.
Compared to Alpha 3 we got tons of bug fixes of course and more than a thousand more binary packages backported from Factory. The biggest missing component right now is Plasma 5.8 as announced earlier².
The amount of build failures went down, thanks everyone for helping with that! Still we have quite some to resolve. As usual help is appreciated to fix the remaining ones³.
Beta 2 is only three weeks away, package submissions for Beta 2 will be considered until Sept 15⁴.
 https://en.opensuse.org/openSUSE:Submitting_bug_reports  https://lists.opensuse.org/opensuse-factory/2016-08/msg00455.html  https://build.opensuse.org/project/status/openSUSE:Leap:42.2  https://en.opensuse.org/openSUSE:Roadmap