Just finished the upgrade. Everything is fine.
I was a bit concerned because I have a dual-boot system and normally run Mint 17.2 as my main OS.
I didn't do any backup, just had to babysit my machine as it went through the upgrade process to make sure it didn't automatically reboot into Mint.
You had better luck than I did. My multiboot (with Mint, Ubuntu, Debian, and Windows 7), downloaded Windows 10 completely then refused to continue. I unplugged the drives that contained the other OS's, and had to download everything from scratch. That's a lot of chugging, but then it finally worked.
This reminds me of when I had to reinstall Windows 7 on this machine recently due to a bad drive. I was finally at the point of installing the 200+ updates and it borked I guess because one of the updates didn't like GRUB, and I had to unplug the "enemy" drives and repeat those hours all over again.