That kind of happened to me, and a few people I work with. Told me the upgrade was scheduled and I delayed it, then didn't get to the next delay in time then it upgraded me. A little bit annoyed as it happened in the middle of a work day so I was locked out for a while as the upgrade occurred. Could have cancelled it but was considering upgrading anyway so no big deal, but I can see how some people would be pissed as they might have thought they were just closing the upgrade window as usual and not realising they were effectively giving their consent to upgrade. If you were away from your PC (and it was turn on) for a week and got caught up it the auto scheduled update then you could very well find yourself arriving to an unexpected and non-consented update. That would suck.
My experiences with 10, which I've had for about 3 weeks now:
All software etc worked fine, desktop and icons and desktop saved docs etc were as they were before so I could resume working immediately. That was good.
Definitely slower to boot up and wake from sleep (and my laptop is no slouch, i7, SSD, 8gb ram etc) but I can live with that. Extra seconds feel like an eternity at first when you're used to very fast boot times. I don't notice it much now.
Excel has crashed lots of time which never happened with 8.1.
Also had a very annoying bug which opened the notification centre every time I clicked the touchpad, which basically rendered the laptop useless as the notification centre popup (from right of screen) action prioritised over what I was trying to click on, which meant I couldn't do anything. Fixed by disabling 3 and 4 fingered touchpad gestures in control panel and restarting.
And when I tried to set up outlook app for downloading emails etc that didn't work, but in the process it converted me to a windows account which was very annoying, but 15 mins of googling and I found out how I could switch it back to a local account, so no biggie.
Also a few times when I close the laptop to put it to sleep it will shut down and restart, loosing document data since the last autosave. I think this is because it's updating, have to look into this.
Other than that I spent 5 mins making it look like windows 7 (did the same with 8.1) by deleting those fucking app tiles or whatever they are called on the start menu, so for me it's functionally identical to windows 7/8.1 Now I get to settings by left click on start icon, whereas 8.1 was right click, whereas 7 was left click etc. I pretty much use the pc as I did for 7 and pre-7 days, so haven't explored the tiles/metro/app stuff at all, or voice control and all that.
But it's fine apart from the issues mentioned above, which I assume will be smoothed out in time/updates. I'll keep 10 (you have 30 days to revert to previous version) as I figure I may as well stick with the latest version if it's functionally identical and I'll have security updates for longer before needing to adapt to a new OS version from lack of support.
I say upgrade if you want to, it's honestly not really any different experience at all to 7 or 8.1. Or don't upgrade if you don't want to. Either way it's pretty much the same. Some people I work with have had a similar experience to me, some have had no bugs/issues, and some reckon it's a big improvement.
I'd like to try something different one day (OSX, Linux etc) but it's just not an option with everyone at work using Windows and the need for multiple people building/working on the same documents and files. I can't be wasting time with file compatibility issues, there aren't enough hours in the day as it is. Maybe if I ever retire I'll try a different OS lol.
So yeah, you could always upgrade then revert back within 28 or 30 days if you don't like it. I know one person who reverted due to some software not running well on 10, and they didn't have any problems with the reinstall of the previous OS version. It was like they had never upgraded. So that's good to hear.