I have found it really unstable, e.g. hangs at shutdown (and this is with no additional software installled!!!!), long lags in the UI/explorer. Dogs dinner of a UI, with a few nice icons but the rest of them (white outlines on black in tray etc), hopelessly unclear. Spelling mistakes all over the place. It feels like a pre alpha, nowhere near beta quality and a million miles from a Summer RTM.
I only noticed some weird translation issues in a German build (10074)... like, for example, "Power" on the Start menu reads "Leistung", which is not entirely wrong but I think not appropriate in the context of shutdown/restart options ("Leistung" refers more to performance than energy, probably it's machine translated?). "Energie" would have been better I guess but it still sounds weird to me. For example, the tool tip of the power button on the Windows 8.1 Start screen reads "Optionen für Ein/Aus"... but that's too long and not perfect as well. What about Windows 95 style and label it "Beenden"? Like in Start -> Exit.
This build is very buggy, taskbar disappeared many times, couldn't close browsers and programs many times, had to do the "famous" ctl+alt+delete to reboot. Wish we have a better build soon!
Build 10074 is now stable, but spartan can be better, it is just a way better then it was but it can be faster and better!
Install your system latest chipset and graphic drivers. This build is more stable than the others I have tested.
Another annoying thing is that I have to click twice on an icon in the taskbar to select it - no matter whether it's a active window or just a pinned icon. I played around but was unsuccessful to discern a particular pattern though. And I have to re-plug some USB devices after a restart.
How is game stability and performance with nVidia drivers for Maxwell's and Kepler's? Do you need to clean-install each new build or can you just download updates to get the latest build? Having to install each new build would not make it a good gaming OS... for now at least.