So, my opinion on where we're at: Windows 10 on the SP3 is excellent overall, with three exceptions.
1) Sleep just isn't reliable, particularly when closing the keyboard cover. I don't believe it's actually sleeping, I don't depend on it to be doing so, and so I use hibernate when I want the machine to be safe for travel and to be locked when starting it back up.
2) Battery life is okay, but not the best I've seen on Win8.1. I expect it to be improved when debug code is removed (along with general performance issues).
3) The biggest stability concern is with various Windows apps, including People and various 3rd party apps. I expect those to be fixed via Store updates once an RTM candidate is released.
But, in general: I like Windows 10 considerably more than Windows 8.1. It's comfortable to use as a tablet, and gets rid of those odd Windows 8.1 quirks that just got in their own way. It's not perfect, but it's an impressive stab at melding the traditional "desktop" OS with a functional mobile OS. It won't turn people off the way that Windows 8.1 did, but it offers real value going forward. And, it works for everyone--it's certainly the best traditional "desktop" OS I've ever used, beating Windows 7 and OS X (which right now is a hot mess of weird gestures and full-screen madness) hands-down. The only place it doesn't work very well yet is on my Dell Venue 8 Pro, but that's not necessarily the UI. It's just not terribly stable on that machine.
I think Microsoft did something amazing over the last year or so. They deserve some serious kudos.