I have successfully installed the core Visual Studio 2015 and Windows 10 tools. However, while trying to install emulators for Windows Mobile 10.0.10069, the installer stops at around 80% and never proceeds forward. The screenshot shows Task Manager too.
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Was happening to me on both 10166 and 10240 builds. On the 10240 build I opened task manager after waiting quite a while and killed (Vs)SecondaryInstaller. The VS2015 main installer continued immediately and finished without errors. Later did a repair of the VS Installation just to be on the safe side.
I solved the issue by using web installer. Even this web installer may not work well with the dev machine. Solution is to run installer on another machine (Windows 8.1 works. Any other Windows should work too.) and choose download option. Then move the downloaded directory to dev machine and install emulators from that directory.