It's a free upgrade because of data mining included with Win10 (data is huge and targeted ads are huge income). There's a lot of privacy issues with it but I don't think it's anything we haven't seen in OSX, Android, GMAIL services etc. The problem is that Win10 installs with it TURNED ON and you have to turn it off if you don't want it. Kinda annoying and a little sketchy.
It's better than 8/8.1 because that crap metro is outta the way but metro UI in Win8 can be stopped will Classicstart/Start8.
Win 10 feels a little snappier than 8 and definitely snappier than 7. It's obviously been polished in that regards.
The most important part of Windows 10 is the DX12 API that will translate to better performance for both AMD and NVIDIA GPUs for the future. If anyone ever used Mantle with AMD (BF4 as an example) it will add performance increases like that. That's really the only reason to upgrade to Win10.
I use 8.1 at work. What I'm currently trying to do is run a virtual Win10 with PCI passthrough and KVM inside of linux (think baremetal hypervisor). If I can get that to work then I'll stop dual booting. Right now I haven't been able to get Win10 to work but it hasn't been out long.
Honestly, if it wasn't fort he DX12 API I would stick with 8/7.
