2011 MacBook Pro, Yosimite, 500GB SSD
I had WIn7 installed on a 100GB Bootcamp partition, booted into Win7 and upgraded to Win10 last night. Everything went fine with the upgrade, but the touchpad wasn't working. I tabbed around and couldn't find the Bootcamp utility anywhere to reboot into OSX or the folder to try to reinstall Bootcamp drivers. I tried restarting using various Mac shortcut keys for the Boot Manager, Recovery Manager, or Target/Alternate Disks, but Windows always came up. It appears that the Win10 upgrade completely wiped Bootcamp and the dual-boot capability. And of course my initial search to install the Bootcamp drivers/utility directly on Windows came up empty, with Apple only providing the option to download and run Bootcamp within OSX. This is still the early stages of my search though, so I may still find what I need to install on Windows.
So for now, beware about upgrading a Bootcamp partition in place. It might be better to download the iso from another Win10 computer and use your Product Key to install a fresh Windows from OSX onto the Bootcamp partition.