So I've gone ahead and installed Windows 8.1 Preview on several machines at this point. The installs went smoothly as expected. There were a couple of hiccups, but nothing that was a disaster, yet. :) Below is a list of the machines I've installed the Preview on. In all cases, I chose to Upgrade and keep my existing Settings, Apps and Programs.

Warning : Installing anything called "preview" is for advanced/enthusiast folks. Please be responsible and don't install the preview before you have made sure there's nothing on the machine you can't afford to lose. In the unfortunate event of "Something Bad Happening", be prepared to re-pave your machine to get back to status quo.
  • Surface RT
    This was the easiest by far, used the preview site at http://preview.microsoft.com, installed the update that would allow the upgrade to 8.1 and then sat back and waited. It does take roughly 2 hours to download the update and apply it, so have some free time and patience,
  • Surface Pro
    Something was flaky here, the preview site just wouldn't work for me, I don't know why. So in the end I went to my MSDN Subscription and downloaded the Windows 8.1 x64 ISO and ran setup from there. With the built in capability of Windows 8 to mount an ISO this was very easy. Once I got ast that part, the upgrade ws similar to the Surface RT.
  • Dell XPS 12 ( Laptop )
    This is still in progress, but running smoothly as expected, waiting for the final configuration to be completed.
  • Parallels Virtual Machine on a MacBook Air
    Talk about disasters, I started the upgrade using the upgrade site, it got about 3/4 of the way there when the SSD in the MacBook ran out of free space. I had to stop the VM in Parallels and that was the end of the VM, it will no longer start because Paralells cannot find the HDD. I am not blaming any of this on Windows 8.1, but purely on the lack of stability in Parallels to ensure my VM integrety. This will give me an opportunity to try a fresh install of Windows 8.1 and see how it differs from a Upgrade. My assumption will be that it is not that much different, hopefully just faster.