I can't really comment on whether Win 10 mobile is ugly or not since I haven't installed it; but I would hope all the complainers here have made concise and specific comments on the Insider input. Otherwise it's just pointless ******** on a forum that has no influence on the future outcomes. I have installed the preview on a desktop and the progress in the UI and functionality have been pretty impressive, hopefully the progress will translate to mobile as time goes on.
It may of changed since I last successfully opened the insider feedback app, but much of the Insider comments were more on harmless UI aspects of WinMo 10 (Round profile pictures, for example) than the major aspects (Lack of pivot, for example). There was a lot for the hamburger distaste, though.
I personally find WinMo 10 disastrous as it currently lacks many of the features I used on a daily basis (Me tile, Integrated Social Networking, Photo Folder Albums, etc.), and the interface is much more polluted with 'chrome', as one may put it.
I've long said Microsoft gave up with the Metro design principles (fluid, clean, content-focused, chrome-light) around Windows 8, and Windows 10 is making that clear for me. While I cannot speak for the market as a whole (as maybe a majority of those flocking to the OS from Android and iOS like these new changes), I can say without a doubt that the UI of WinMo 10 is quite awful for my tastes.
For instance, I honestly loved the old Settings menu design, with each title having darker, smaller text bellow it detailing the options contents; Granted, it needed to be categorized (hard to find much without scrolling), but it was clean and efficient. I also enjoyed the old app bar, as I could drag my thumb up in a forward movement to open the menu, and the icons looked like buttons, giving a clear area I needed to touch; The new app bar requires me to tap, not drag, the ellipsis, which has become quite jarring for me, and I always worry I will mis-tap the wrong icon.
I also take general issue with the added chrome of unnecessary icons and headers (a contact's name now takes up 2x the space in a messaging chat), as well as the interface scheme in general (it looks too much like Android ICS' "Holo UI", sans the blue font). Strange though that the design really feels different from Windows 10 itself, aside from the icons. Perhaps the design just feels different on a PC, though. Speaking of which, the Cortana UI in general is even strange. It feels like each "section" of the interests she pulls is a piece of paper overlaid atop the ui, and it once more is a jarring experience. The WinPho 8.1 Interface was much cleaner and stylish, and honestly fits in more with some of the new Windows 10 design elements than the Windows 10 UI for her.
Overall the experience really screams late 2011 Android, and lacks many of the enjoyable functions and elements that were the reason I chose Windows Phone in the first place.
I bought a 635 recently as I was in desperate need of an LTE device, but I can say for certain that unless Microsoft scraps the current designs in favor of something better, then my next phone is going to be of the Moto line of devices. Material Design really feels like Metro design reincarnate, and Microsoft has stated the Cortana shall eventually be coming to the platform, so there is not much to keep me on the Windows bandwagon.
And I highly doubt Microsoft is going to make a U-turn this late in development. Even with WinMo 10 having a later slated release date from the Summer launch of Windows 10, it's still due sometime this year (afaik), and considering the enormous design changes made already, I feel it is unlikely they will undo all that work.
These are just the personal opinions of an "armchair designer", but none the less, I feel many agree with what I said, mainly those being early adopters of the Windows Phone operating system (during both it's 7.x and 8.0 days).
Random thought, but maybe Microsoft will finally offer enough APIs that the original experience of Windows Phone 8.1 can be duplicated for some functions (Messaging, Mail, People, etc.)? At least the third party developers could then give those who admire the Metro design language something to work with. Perhaps they could even improve on the original experiences?