Windows 10 is for sure an overall improvement to Windows 8 and Windows 8.1. However, there are a lot of features that I personally don't admire, such as the tracking features. I would prefer Windows 7 over Windows 10, but over all Windows operating systems, I would use Windows XP. However, aside from Windows, I would definitely use Linux.
I haven't noticed any slowdowns performance wise since I upgraded to Windows 10. In fact, I find it to be a lot faster than Windows 8.1 when it comes to things like booting up and opening programs. Overall, it just feels more snappy and responsive. However, the spying and other features like integrated Bing, default metro apps, and Cortana really make Windows 10 quite annoying.
For me i had some wifi driver problems (keep disconnecting 0_o) and had to go back to windows 8.1 again ......... still have windows 10 installed at work and there im Happy with it............
Performance : Faster startup
Things that suck : Glitched Shearing and network center once and searching .....Not that great
Mostly the windows 10 upgrade is not a bad choice ...... most of my clients are happy with it =)