I installed it on a lightly-used PC a few days after it was released.
I fitted a spare hard disk, as the PC already had Windows 7 on it. I installed W10 to the new disk. I didn't attempt any fancy boot managers, I simply decided I'd go into the bios and change the boot order of the disks if I wanted to switch between 7 or 10.
However, it didn't work as expected.
Installed OK, booted into Windows 10. Played around with it. Shut down.
When I next booted it, it started in Windows 7, but complaining that it needed to run CHKDSK, which it did. The previous shutdown (either from 7 or 10) had been "proper" - there should have been no reason for chkdsk to want to run.
Anyway, I used the Windows 7, then shut it down.
When I rebooted later, I got Windows 10. Which complained chkdsk needed to run!
Shutdown later, start it up later still.. I get Windows 7. Which complained chkdsk needed to run.
Up until today, that is what happened. One Boot I'd get 10, the next I'd get 7, then 10 again.
Today it booted W10, told me there was an update to install, so I did that. It shut down, rebooted Back into Windows 10 which told me it needed to do a chkdsk, and that it might take up to an hour (hadn't seen that before). I left it chugging away, came back to a black screen and no disk activity. Can't ping it on network.
I power it off, restart it, and W10 starts up again. I get logo and rotating circle, then the screen goes black.. and I have to power it off. Tried a few times, no joy.
Have unplugged the W10 hard disk for now, and W7 is booting ok (after complaining it needed to run CHKDSK...)
Ian