So far, I've skipped two - Microsoft Bob & Vista.
If I do move to Windows 10, it WILL NOT be via the "free" upgrade. I'll buy Windows 10 Professional.
Why?
From Microsoft - Reinstalling Windows 10 after upgrade
Q: What happens if I change the hardware configuration of my Windows 10 device?
A: If the hardware configuration of your Windows 10 device changes significantly (e.g. motherboard change) (also network cards, or multiple small changes such as hard drive & memory) Windows may require re-activation on the device. This is the same experience as prior versions of Windows (e.g. Windows 7 and Windows 8.1). The free upgrade offer will not apply to activation of Windows 10 in such scenarios where hardware changes reset Activation.
There are many people who never change the parts inside their computer.
I'm not one of them. Under normal circumstances Microsoft makes re-activation as simple as can be

That re-activation problem also applies to building a new computer. Most don't build a computer, I do.
When I buy a copy of Windows I can install it on a new build (retiring the old one of course). Again, I never had a problem from Microsoft when I did so.

So you took the bait on the "free" Windows 10 upgrade.
Some time later you decide to replace the video card and clone your system over to a nice, fast, SSD.
You're now staring at a screen that says Windows 10 is de-activated. Do you want to buy a software license from Microsoft?

"Free", yeah. Right.

I mentioned all this back on July 4th 2015 - Free Windows 10 upgrade GOTCHA
The "free" upgrade isn't all that bad for some people. But most will find a bill to pay some time latter.