I upgraded from Windows 8.1 to Windows 10 yesterday. I have Windows on a separate drive in my Mac Pro (please see signature for details). My Windows 8.1 install was a standard Bootcamp installation (as opposed to an EFI installation). I also run this installation in VMware Fusion.
When I clicked on the upgrade option in the task bar, I was initially told that it couldn't be done and cited the VMware video drivers as the culprit. A quick search and I found out that I would have to download the ISO, burn it, and install from there.
Once I go the Windows 10 ISO on to disc, the installation process went without a hitch. I chose to keep all my apps and settings and it did a good job of doing so. I was also careful to turn off all the privacy invasive stuff that Microsoft has on as default. After the installation was done, I was left without any Nvidia drivers for my video card so my system was running something like 1024p resolution. I downloaded the latest from Nvidia, installed, and got back my 4K resolution.
Next, I booted back in to OS X and ran it in VMware Fusion. I reinstalled the VMware tools and everything is working great.
So far, everything seems to be running perfectly. I haven't experienced any anomalies or issues at all. it took over 12 hours before my copy of Windows 10 was "activated" but now it is and all is fine. I'd have to say that overall, it went smoother than I expected it to.