Just wanted to share my experiences. I upgraded 2 Dell OptiPlex 9020 from Windows 8.1 to 10 and one Mac Bootcamp from 8.1 to 10.
The Mac was a little problematic, the upgrade went OK and didn't take very long - 30-60 mins or so, but I had no activity on the Start button and other options - notifications, file explorer. The right clicked work so I could launch a cmd prompt as admin and ran PowerShell and then

Get-AppXPackage -AllUsers | Foreach {Add-AppxPackage -DisableDevelopmentMode -Register "$($_.InstallLocation)\AppXManifest.xml"}

This fixed it on reboot but the problem returned after another reboot. So I clean installed which was a pain as I had to delete the bootcamp partition, remake it then install Windows 8.1, then activate it and then upgrade it to 10 from the DVD I downloaded. But when I final got there all was working and has been ever since.

The Dells were no problem, I made an image backup first so I could roll back exactly if I needed to but it wasn't neccessary, both Dells upgraded without a hitch and are both working fine.

I had one little issue with Outlook 2013 on all 3 machines: Mail would not send, mail just sat in the Outbox. I ran sfc /scannow which fixed the issue, didn't even need a reboot, just opened Outlook after the process completed and mail sent as normal.

I like the OS. An improvement on 8.1 for me. I have to be honest and say that whilst I didn't mind the Metro screen and I customized it heavily, I bypassed it most of the time and I found the charms a pain when trying to find a setting, would it appear or not, and what was in there once it did appear? Control panel? No! to get that you had to be on the desktop and get the charm to appear, if you did it on the Metros screen the contents were different!

Anyway that's a thing of the past, the new Start menu is much better, for me. One last comment:

I use both Windows and Mac OS depending on what I'm doing, some programs work better for me on Mac - like music stuff, and other things better on Windows, Excel for instance which I can't use on Mac - it's so tiny for a start! Outlook will change signatures automatically depending on which account I'm sending from on Windows but not on Mac - so annoying!

I had occasion to boot into Mac Yosemite on the Mac Pro and was immediately struck with how dated it looked, like a kids toy with icons bouncing up and down at the bottom, and Mac OS always used to look and feel much slicker the Windows.

Anyway I hope that helps if you encounter those same problems.