This thread has now been closed to direct all further updates about this release onto the new RC2 announcement:
Thanks for all your continued participation in the release candidate process!
Please file a bug for any new issue you find in these Cycle 6 versions that you would like the Xamarin team to investigate. This Alpha release is a chance to help ensure that the final Cycle 6 Stable release will work smoothly with your specific projects and environment.
You can install this new Alpha version or switch back to the current Stable version by changing updater channels.
If you prefer, you can also use a special version of the standalone combined Xamarin installer that automatically downloads all of the Alpha packages and installs them. Using this installer is equivalent to switching to the Alpha channel, except that your updater channel preference can be left at "Stable." See the release blog post for the link for Mac and Windows.
Non-public Bug 35425 - [Standalone Alpha installer] Mac-only: on some systems the standalone Alpha installer might crash on the second installation screen during some attempts to run the installer. Workaround: wait approximately 10 seconds on the first screen of the installer before clicking "Continue".
Visual Studio Extension
Release Blog including estimated timeline to Stable: https://releases.xamarin.com/alpha-release-cycle-6-preview-update-release-candidate-1/
[XamarinVS] [iOS] XamarinVS 4.0.0 uses a new SSH-based build host connection that no longer requires the old "Xamarin.iOS Build Host" app on the Mac. (For a few more details see the release notes and a short introduction to setting up the new SSH connection.
[iOS] The Cycle 6 builds now require Xcode 7 or higher. For example, attempting to build an iOS app for device using Xcode 6.4 will now fail with "Native linking failed, undefined symbol: ___divdc3".
[Xamarin Studio] [XamarinVS] [iOS] [Android] Support for OS X 10.9 Mavericks has been deprecated. The Cycle 6 builds now require OS X 10.10 Yosemite.
EDIT Oct 6: Update link for "short introduction to ... the new SSH connection".
EDIT Oct 30: Update for new Alpha builds!