I have actually been using Windows 10 for over 6 months (before official release).
I actually like it and would say to anyone running 8 and 8.1 today to upgrade,
( because if you think 8 och 8.1 is good you are first crazy and second wont be disappointed in Windows 10 which imo is far better then 8/8.1 )
Windows 10 is still a work in progress and you will notice that from time to time but whats the alternative?
8/8.1 are work abandoned imo, Microsoft rushed so much at Windows 8 that they never catched up even with the "upgrade" try of Windows 8.1... and then came Windows 10.
But there's also a lot of good things with Windows 10 imo, most of them making it easier for IT-administrators to deploy and manage (what i have noticed in my line of work).
I have to say I think Windows 10 runs even smother than Windows 7, they changed the look of a lot of things that made me furious in the beginning but I got used to them and have started to even like most the changes.
You might have to throw a tweak or two in but all in all drivers, software etc works flawlessly. You just have to give it a go the make up your mind for yourself.
Privacy is as always, turn it off by deleting files, services, registry keys etc. All the web is going the same direction of collecting your every fart and burp.
Honestly for me switching from Windows 7 to Windows 10; now i would never downgrade.
The upgrade part takes around 3 hours not counting the downloading part depending on hardware.
If you're deploying to multiple computers maybe sccm and/or mdt deployment would be worth looking at as it provides unattended deployments. A clean reinstall to Windows 10 with all patches, updates and software from referance image takes about 1 hour and 1,5 hour if you use USMT.
Yes Windows 10 is free for everyone having Windows 7, 8 and 8.1 until the summer of 2016.
And Microsoft said that Windows 10 will be the last OS, after that only updates to Windows 10.
Theres also rumors (or confirmed idk) that Windows will be "as-a-service" monthly/yearly subscription.
Buy honestly like every windows feature and version in the past this will be hacked providing ppl this for free im sure.
These are my 2 cents about Windows 10.
Cheers