Windows 10 free upgrade period ends the day after tomorrow, today being the 27th.
Windows users, I recommend you to "upgrade" not because Windows 10 is a good OS, but because it gives you a free copy of Windows 10 for the computer you're upgrading then you can use your existing serial number (of Windows 7/8) and install it elsewhere.
I did this with all of my computers with a single copy of Windows 8.1 Pro I bought super cheap (via Microsoft Volume Licensing) for something like 15USD; then went ahead and one by one I installed it and upgraded it on all my computers, most of my computers are Macs, I only have one actual Windows PC. Then waited for it to be registered with Microsoft and did a clean install of Windows 10 right after just to verify it'd activate.
This is a great hack since technically I can use Windows on a single computer at the time, so I did not violate any term of use or whatever and now on every computer I own Windows 10 can be installed from scratch if I wish so (I won't.)
The good news is that if you're running Windows 7/8 and upgrade you have I think 30 days to rollback. So, you can install, wait for it to activate (let it do a Windows Update for to be on the safe side) and then rollback and delete its files.
The bad news is that Windows 10 updates itself no questions asked. So it might not be a good idea for mission critical computers like NVRs because of downtime and Microsoft if know to push bad patches. I mean, good-written software shouldn't need patches, fixes and whatever, but whatever. The second batch of bad news is that Microsoft collects data on you whether you like it or no, even if I'd get fed up with OS X (coming soon macOS) I'd still wouldn't switch just because of that, I've said it a thousand times, I don't care if it is for the greater good, they should never force you to send data. Unknowingly so.
Basically you're improving Windows with that data, but the thing is, let's say you paid for Windows, you shouldn't keep paying for their Research and Development, which is what's happening behind the scenes when you're using an sending telemetry data to Microsoft.
Mac (OS X, macOS) asks you upfront if you want to help or not right when you create your user account. Windows misleads you with a big EXPRESS SETTINGS button when you set up you COMPUTER, NOT YOUR ACCOUNT, so you only have one chance of seeing this, and that Express Settings button will set all settings that benefit Microsoft, not the user. The alternative to that button is a tiny text link, lost in a sea of similar looking text on that page, sandwiched right in between the text so it is harder to see.
The third set of bad news: Windows 10, that software you paid for comes with advertisements built right in the OS, from the Start Menu to the Lock Screen to the Office365 offer, it's so fucking annoying. Again, you're forever paying for stuff Microsoft sells you, just trash you computer and get a Mac, it's a much saner choice.