Bumping this thread...
I upgraded to Windows 10 today, from 8.1. The upgrade process was smooth and most of my settings from 8.1 carried over automatically. It's pretty easy to disable the automatic updates and turn off the telemetry/spyware, but you do need to read some articles to know exactly what to do with the registry, disabling services and so on. Some obscure drivers I was using on 8.1 like VSTMIDI and Lightscribe still work fine. Initially it seemed very good. However, one big problem is that custom themes and theme patchers are much harder to get working, and are specific to different versions of Windows 10. I had Classic Shell and a custom dark theme in 8.1, and never used the Metro UI at all. The former works great, but I can't get any of the 10 themes I'm finding, or my old 8.1 theme, to fully work. I'm using a custom high contrast theme for now, which is easier on the eyes but is based on the Windows 98 theme system and is ugly compared to an msstyle-based modern theme. There is a "dark theme" option in the 10 control panel but it only affects the Metro-style programs.
Also, a lot of older games are broken in some way. Most DX8 games which worked great in 8.1 have problems, with some refusing to start at all, some only running in a window, most not working with driver-level vsync (so they run at 1000+ fps, which often changes the game speed), and some having a weird glitch with mouse movements.
Some of these are from GoG, so there might be hope of getting them fixed. On the flip side, the 30fps problem that affected most DX3-7 games in 8.1 has surprisingly been fixed, so several even older games actually work much better than they used to in 8.1. DX9 and up generally has no issues.
I'm surprised how different 8.1 and 10 are in terms of game compatibility. I might try a different Nvidia driver (it's the same version I had on 8.1) but don't want to spend a lot of time fixing everything. I have a separate XP install for such games (although it's a hassle to boot into), but the lack of themes is a real showstopper. If I can't get that fixed, I'm going to roll it back to 8.1.