My experience so far:
They're batting 1-for-3 on my three Windows devices. I have a Nextbook 10.1 Intel Atom Bay Trail tablet that I was really hyped to put Windows 10 on... but within 24 hours, I rolled it back to 8.1. It's sluggish, buggy, and they really cut out a lot of the
good tablet features of W8.1. I really should have done my due diligence; When my frustration with the shitty navigation in Tablet Mode and Desktop Mode (on a tablet) reached a boiling point, I went searching the internets and found
this official community thread that has been active for the last 6 months highlighting the problems with the tablet side of Win 10 thoughout each build. All of the issues listed in the RTM build are still present.
I have a Windows 8.1 laptop that I use sparingly. Since it really doesn't have that much on it, I went ahead and did an in-place upgrade. It worked like a champion, though it did take a
very long time (which I don't understand because I didn't have very many apps or legacy programs on the system at all). After it finished, the system works just fine, the desktop interface is crisp, clean, and enjoyable to use. I still don't like the division of "Settings" and "Control Panel", but I just made a tile for Control Panel since I can do everything I need to do there.
For my main machine - which, for the sake of clarity I'll refer to as a production machine - I wanted to dual boot Win 10 alongside Win 7 Pro that I've been using since Win 7 went RTM. As anyone who has supported Windows knows, doing an in-place upgrade of a brand new version of Windows on a production machine is asking for trouble and it's better to wait for bug fixes if not the first Service Pack. Unfortunately, Microsoft, in their infinite wisdom, made a direct custom install impossible. I'm sure the convoluted process has been touched upon in this thread, given the thread is 130+ pages long, but for those just tuning in I'll summarize: You basically have to do an in-place upgrade in order to upgrade your product key, retrieve your product key, roll back to your previous OS (and pray it kept everything exactly as you had it),
then do a custom install with a bootable version of the ISO. The process is beyond asinine. I'm not going to do an in place upgrade just to roll back the system - that's asking for double trouble. There's no reason for Microsoft to
not include a secure system that generates a Windows 10 key based on your existing product ID and hardware hash for a Windows 7, Windows 8, or Windows 8.1 machine.
Out of three machines, the only one with Win 10 is the one I use the least. Blah. I have to say, I'm not very confident about Windows Phone 10 at this rate (especially since I found a similar thread on the MS Community boards outlining all the problems with the latest builds of WP10).