This thread has now been closed to direct all further updates about this release onto the new Stable release announcement:
Corresponding Release Blog post: https://releases.xamarin.com/beta-preview-cycle-6-preview-update-release-candidate-3
This is a feature release. Two of the key highlights are:
New SSH-based build host connection for XamarinVS. The Mac build host connection for XamarinVS has been rewritten to use a new SSH-based login that simplifies certain connection scenarios and solves several bugs with the old build host connection. Note: The "Xamarin.iOS Build Host" application has been intentionally removed in this version because it is no longer needed for the SSH-based connection.
Mono 4.2.1. Xamarin.Android, Xamarin.iOS, and Xamarin.Mac have all been updated to use Mono 4.2.1. This Mono update replaces several major components of the .NET runtime with Microsoft's open sourced stack, which brings many bug fixes and performance and compatibility benefits.
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 Beta 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 Beta 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 Beta packages and installs them. Using this installer is equivalent to switching to the Beta channel, except that your updater channel preference can be left at "Stable." See the release blog post for the link for Mac and Windows.
Release Blog post including estimated timeline to Stable: https://releases.xamarin.com/beta-preview-cycle-6-preview-update-release-candidate-2
[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] [Xamarin.Mac] Support for OS X 10.9 Mavericks has been deprecated. The Cycle 6 builds now require OS X 10.10 Yosemite.
Nob-public Bug 35797 - [XamarinVS] [Xamarin.Forms] Hang/freeze when attempting to create a new Xamarin.Forms project from template.
Non-public Bug 34828 - [Xamarin Studio] – Holding down Command+Z batches undo actions. (Undo is not executed until key combination is released.)
EDIT Nov 13: Update Xamarin.iOS, XamarinVS, and Xamarin Studio build for additional bug fixes: "RC3". Remove fixed known issues: 35555 35639.