I've been using this on my 920 for a week or two now. If I were to rate it, I'd probably give it something like a 6/10. I don't feel like much of anything is improved from WP8.1, but a LOT of things are worse. Some key things:
--I didn't think the last iteration of Xbox Music could be made into a worse app, but Groove Music has pulled it off. Loading in new music isn't any faster than it used to be, in my brief experience. They took out the artist-specific song view, a feature I used a LOT. The pop-out menu is trash, and conceptually awful for touch-based navigation. It feels like something met for a mouse. Having the Now Playing thing follow you through menus is cool, right up until you want to select something at the bottom of a screen, and it partially blocks what you're trying to reach. Oh, and don't forget that the live tile doesn't work and the lockscreen functionality is apparently gone. Given that the music player is BY FAR my most-used app on mobile, these problems are a major deal to me.
--E-mail is horrendous on Windows 10. Why is everything thrown into Outlook, which I have no interest in touching with a 10-foot pole? I liked pinning my individual e-mail accounts with WP8.1, while also having lockscreen notifications for them. Now, it's all a giant mess in one tile, and I have to use the stupid pop-out menu to navigate between accounts. Productivity is down in e-mail, to the point I honestly don't even bother checking my e-mail on my phone much anymore. I used to get my alerts and check stuff ALL THE TIME on my phone, but it's such a chore I can't be bothered with it anymore.
--Is there some way to change text tones in W10M? I didn't see it, and the alert sounds are SO BAD in this OS. I hate getting texts because my phone is so annoying now. What's more, after having different alert tones for different apps, they're all the same, and I hate that. I don't know if I got something important or trivial on my phone.
--The Outlook Calendar app blows. I don't want something tied to my e-mail. I want the old app back. For starters, the app list icon is broken, so it's an empty tile. Switching between months is tedious because of the touchy flip controls that make no logical sense. Oh, I want to go to the next month? That's too bad, because slowly swiping up doesn't send me to September, it sends me to next January.
--Multi-tasking is worse as well. For some reason, they think that backing out of an app means it should go into the multi-tasking buffer, rather than closing it. Too often do I want to get something I deliberately suspended back up, only to have it buried behind 3-4 things I tried to close, but they were put into the multi-tasking window. Oh, and why was it moved from the left to the right, to copy Apple? Pointless visual change that makes usability for loyal users worse, classic Microsoft.
--The browser's much worse. It loves to open tabs for no good reason, leaving the tabs pane a mess. I hate it SO much. Plus, I didn't see a way to set a home page, and that idiotic "Browser for Doing" default page just makes me hate Edge all the more (don't get me started on how disjointed and weird it is on the desktop).
--The Xbox app seems to be worse than the Xbox One SmartGlass beta app. I uninstalled the latter, thinking the former would be the go-to solution for connecting to my One. Now, I might go reinstall the SmartGlass beta because I hate the Xbox app so much.
--The Glance app functionality isn't working, so I no longer get weather info on my lock screen. That's quite annoying.
--Why can't I swipe to the right to dismiss toast notifications as the appear anymore? I guess convenience is the enemy of W10.
--All of this leads to one last thing that the previous 8.1 update screwed me with: TOO MANY APPS. They've broken everything down into a system app that I can't stand. I don't want 15 pointless apps in my app list. I don't need a Cortana app, Cortana does nothing for me (literally the only thing it's done for me is get rid of the lockscreen-to-Bing shortcut, which was another backwards step for the OS, to me). I'm not going to call customer support, so I can't see why I have the app. I don't listen to the radio, so that app could gladly leave. I count 10 system apps on this OS that I want to remove from my phone, but can't. It's Android-level clutter, and I hate it.
There really isn't much to say about W10M that's an upgrade. I guess the built-in flashlight is nice, but it's strangely NOT an app I can pin. Dragging down the Action Center to use the flashlight's kind of annoying. The new, bigger set of quick-action icons would be nice, if it were able to be fully customized. There are 16 things in the expanded quick action area, but only 5 of them meaningfully interest me. I'd like if I could both hide the undesirable ones altogether AND move the "All Settings" one back to the bottom-right from the top-4 action row. Tucked in with the rest, it's an inconvenience, so I instead pinned a tile for it and use that. That the new Maps app includes 3-D maps, a la StreetView, is nice, but not a frequent-enough feature to really move the needle for me.
Overall, W10M is a cluttered mess. It's killed so much of the WP convenience, especially with the switch from swipe-based navigation to pop-out menus. If you could AT LEAST re-position them to the right side of the screen (preferably with the pop-out button in the bottom-right), it wouldn't be as bad. Instead, even a small-ish phone like the 920 almost requires two-hand use, thanks to poor design. I feel like I'm just using a slightly better version of Android, but with fewer apps, at times. Navigation and multi-tasking are clunky, the music player's a pile of steaming ****, and it's all quite frustrating.