Paul Thurrott on build 9879:
Obviously, there are some nice improvements here—and the seismic shift that’s happening with OneDrive—but the big news here, perhaps, is that this is the first build that isn’t stable or reliable enough to use regularly. And unless this is fixed soon, I may have to take the unusual step of moving back to a previous build, or to Windows 8.1.
Developer builds have bugs and certain instabilities. That’s expected and totally fine.
Microsoft has announced that it will not be issuing another build until 2015.
Not releasing any updates for about two months? That is almost entirely not fine.