I just loaded a new instance of the TFS 2015 RC and encountered the new build system. I see that MS is positioning it as the new default build system so I am working on recreating my existing XAML builds. I am coming up short, though.
The first 2 runs of my build continued past the Get Sources step but there were errors in the build. After resolving that, my builds now stop after the 'Get Sources' step. There is no error message. My next build step just says 'Not started'.