My cursory comparison of the Windows 10 Upgrade and Apple iOS 8.4 upgrades…The Windows 10 install was super smooth – all apps still working. The only issue I had was a graphics card issue (nVidia GeForce GTX 780) that went away once I updated the driver from the nVidia website.
Now for Apple…not smooth and lots of reboots and resets. I am not happy with the new features (or the useful ones that were removed) that are really designed to make it easier for Apple to gather data on you.
At least with Windows 10 you have the opportunity to turn some of that off before your upgrade begins. Yes, I recommend you do that. Take a second to read what they are telling you before pressing the express install button. Actually, take a few minutes before that and run a backup.