First off Microsoft have staggered the release over several weeks, so not everyone got their free upgrade on launch day (29 July 2015) probably to ease the load on their download servers, but it's no doubt a marketing ploy too. Making people sign up to reserve their free upgrade and then wait in an orderly queue to build up the anticipation around the new release. The quicker you reserved it, the sooner you'll get it.
It's also a reward for keeping your PC up to date. If you can't see the Windows icon in the system tray in the bottom right of your screen, you need to go to Windows update and download the latest updates, bug fixes and security patches, something you should do whether you want Windows 10 or not. Then you can order your free upgrade, sit back and wait. You can also jump the queue by downloading Microsoft's Media Creation Tool. If you have more than one computer to upgrade I'd recommend using an 8gb usb drive and downloading both the 32 & 64 bit versions to create a universal install you can use for all your computers.
So how does the latest version of Windows stack up?
As someone who can remember the DOS era, Windows has come a long way since 3.1 when the managers in our IT department had to be sent on a training course to show them how to use a mouse. The ending of support for XP produced howls of outrage, largely because people were running software/hardware that was no longer being supported or developed. Still after the abomination of Vista (which replaced XP at work!) Windows 7 was the probably best iteration of a pure desktop environment that Microsoft produced. Then of course came Windows 8, trying to do something different, bridge the divide between the desktop and mobile, and seemingly pleasing no one in the process.
I never had a problem with 8 myself, but can see why some, particularly power users, might. It sometimes felt as if it was trying too hard, trying to force the user to use the computer in a particularly way, irrespective of the platform they were using. Windows 10 puts the OS back in its proper place, the background, and lets you get on with it. I don't have a Windows tablet, but the interface design, clean, simple icons, taskbar, etc. I imagine would look pretty slick on one. The install was painless, I've put in on a couple of desktops and a web-book (remember them) so far. It's no resource hog either with all those machines running at least as fast, if not faster, than before.
The only gripe is that all the third party drivers don't seem to be their yet, I've had issues with a couple of games on Steam, but those issues should diminish over time. I also had to do a registry hack (simple enough, but not for the faint hearted) to get Microsoft Money to work. Still I can't really complain seeing as Microsoft discontinued that particular product in 2009. Guess I'm just another XP user at heart.
All-in-all after 24 hours with Windows 10 I've forgotten it's there, which is probably the highest compliment you can pay a computer operating system.