Please see https://releases.xamarin.com/stable-release-cycle-9/ for the details about this release, including the steps to update or downgrade as needed.
UPDATE Mar 14: See the Cycle 9 Service Release 0 thread for the latest versions unless you are using VS 2017.
Be sure to check the release notes for any breaking changes and known issues, but of course additional bug reports are welcome as always.
Please file a quick bug report using the "Cycle 9" new bug form if you see any suspicious behavior in these versions that you wish to report.
You can also check the list of issues reported by users so far for the Cycle 9 Stable channel versions to see if any match. If you find a match, feel free to CC yourself on the bug report as a way to tally your interest in the bug:
(If you have reported an issue that is specific to the latest Cycle 9 versions but is not yet included in the list, the bug might not have been filed using the "Cycle 9" new bug form. Those bugs will be added to the Cycle 9 list as they are reviewed by the team.)
Comment in this forum thread if your question is not related to a suspicious behavior after download, but instead one of the following:
[Xamarin.Forms Previewer] The Xamarin.Forms Previewer requires a 64-bit version of Oracle Java JDK 8 (1.8).
[Xamarin.Android Designer] The Xamarin.Android Designer currently requires Android SDK Tools 25.1.3 or higher and is compatible with SDK Tools versions up to at least the current latest version 25.2.5.
[Xamarin.VS] Xamarin.VS now requires Visual Studio 2013 or higher.
[Xamarin.VS] Xamarin.VS now requires .NET Framework 4.6.2 or higher.
EDIT Mar 3: Add .NET Framework version requirement.
EDIT Mar 14: Add note about Cycle 9 Service Release 0.