I just installed it on an oldish HP
The machine was a rescued chuck out, that had been used with downgrade rights on XP.
I did a fresh install with the HP Windows 7 64 disk. The resulting system was a bit slow and and nowhere near as stable as I'd like.
I used the media creation tool to do the upgrade without any additional media (same company that required you to click start to turn off, so it's consistent).
However the resulting system actually works really well. (2GB RAM, dual core 3GHz).
The interface reminds me of an old version of linux I used a few years ago, and neither the performance, nor the interface have made me want to install classic shell. I tried installing office 97 and the 2007 compatibility pack (the stated requirement is just to make xlsx files work) and it works fine.
(For comparison I find 8 the worst UI I have used in the last 20 years, reminding me of Windows 3.1 and full screen DOS apps in the way it works.)
My plan was originally to use it simply to get familiar with the new system and to keep tucked away somewhere in case I need a specific windows app, however it is good enough that I have been using it as a regular machine.
In comparison I upgraded a Mac of a similar age to El Capitan, and its performance was very poor. (Perhaps MS should offer WIn 10 upgrades free to Macs, including x86 ones now unsupported.)
I still much prefer Macs to Windows, but I think OS X has been slowly moving backwards since Snow Leopard, while Windows 10 is a small step from 7, but a giant leap from 8.
The issue of forcing upgrades, it is wrong to do this. They should put up a warning with the support status of the system they are using and offering the upgrade, on start up once a month.
I think it might have been more sensible to allow users of XP and Vista (I bet lots would love to have a free upgrade forced on them) to have Windows 10 free, than to force it on others.