I upgraded a few machines to Windows 10, and I downgraded all of them within a week. I upgraded two desktop machines: my gaming HTPC (i5-4560 + GTX 960) and my kitchen's touch HTPC.
Gaming HTPC: The biggest problem that I had here is that Windows 10 messes with the resolution when you don't have an HDMI display active. I would commonly leave Plex open, but when I turned the screen back on, Plex would either be zoomed in to the top-left 800x600 section or it would be in an 800x600 window with the desktop showing for the rest. Also, there were times when my remote simply stopped working. I had to unplug the USB receiver and plug it back in to get it to work. The loss of WMC also hurts this machine. WMC also serves as a nice 10-foot UI with proper remote support.
Touch HTPC: The biggest problem here is a real head-scratcher. This HTPC is treated a lot like a tablet. So, I tend to use the built-in split functionality, which means I can have media open while a recipe is open in the other section. One thing you can do is hit the Windows key, select a new Modern app to open, and Windows will allow you to replace one of the current app split panes. In Windows 10, it simply opens as a full-screen app each time and removes your split. To be fair, I think they actually fixed this in a recent update, but it was really strange why they removed the feature that was in Windows 8. Also, Modern apps will only default to full-screen view when you're in Tablet Mode. The problem with that is that Tablet Mode seems to be kind of buggy. The first time I enabled it, a Modern app immediately popped up but never loaded properly. It was the phone settings app or something like that.
I never really had much of a reason to remove it from my two laptops, but I just didn't really want to deal with it anymore. Since I use digital tuners, I can't really replace it on my desktop, or I'll give up my ability to watch protected content through WMC. Yes, there is some hack that can help get it back, but I'm not 100% sold on it yet. I could get around this by just "acquiring" the TV shows that I normally record, but that's technically not legitimate. Once SiliconDust's DVR comes to fruition, I should be fine.
A restore point isn't necessary if you go through the upgrade process since Win10 will save the old windows dir and allow the user to switch back to Win7/8 within 30 days.
Except you need to be rather careful about what you do
after you install Windows 10. Any changes made after Windows 10's install will
not be reverted. You also have to be aware of the difference in drivers. For example, as of right now, the Nvidia drivers are still split between Windows Vista/7/8 and Windows 10. So, if you install the Windows 10 driver, which you will have to, you will still have it when you downgrade. The problem is that you can't really tell that easily, because things will look right on the surface.