This thread reminds me about when Apple made that radical UI redesign in iOS 7. A lot of users hated the design, pretty bad (some users with more age even say that their iPhones looked like a child color book), some others hated the performance on the iPhone 4. Now, 2 years later, and iOS 9 put it together the general idea of the new UI, with a lot of refinements and looks pretty good at my eyes (also, the UX is smooth as hell), OSX also adopt the iOS style in almost every corner of the UI and looks really nice. This happens, everywhere, anytime, when you are happy with the UI that you have on your device and then, in the newer versi?n of the OS, is radically changed and everything you use to know has a new face and you need to relearn how to use it, you like it or not.
The UI of Windows 10 Mobile has a lot of potential, I really like what they are doing in matching a lot of UI elements between Windows 10 and Windows 10 Mobile, but MS is still doing refinements on Windows 10 and the general UI, so Windows 10 Mobile still needs some time to get a more mature UI (just like the WP7>WP8>WP8.1, the same UI design, style, lenguage, etc.). Yeah, it's still a mess with a lot of mixed elements from WP8.1 and W10 and not a congruent UI in between in-house apps (Groove, Maps, Weather, etc., they share the same "idea" for what it should be, but those ideas are not 100% equally implemented, and I'm talking about the infamous Whopper Menu, the constant updates are bringing some nice changes to the apps and making them more alike in UI, but they aren't still right there).
Windows 10 general UI is great, it's still a bit green, but has come a long road from the first time we saw it and the way it's going looks nice, and I say the same to Windows 10 Mobile. At least these two are much more alike in general terms an menues and configurations screens and Start Screen/Menu that Windows 8/8.1 and Windows Phone 8/8.1, so, I give them the chance, MS can make it work really nice.
Still, my mayor concern is the performance. The ones with Windows 10 Mobile Builds on their phones, it would be nice to post their device, the Build number and their general experience with it, the specifically see what devices in what circumstances they work, and where it doesn't.