I'm running a Nokia Lumia 520 (I know, ancient hardware).
I had some issues with Build 10572. It was great but I found a major touch bug that seemed to be associated with the Windows Feedback app. It was reminiscent of the infamous Lumia 1520 phantom-touch but far worse. You could barely use the phone ("barely" is arguable.) Every tap was actually registered as two taps, even for the capacitive keys, and every swipe was registered as a tap and a swipe. Just letting the phone sit resulted in it opening and navigating through apps, even pressing the hardware capacitive keys to go to Task View and to the Start Screen. It was very strange. The first instance required me to roll back to Windows Phone 8.1 (as it wasn't possible for me to get to Settings>System>About to hard reset,) and jumping forward. The second instance was thankfully only temporary and didn't show again after a few minutes of frantic glitches. The moment I saw it was happening again I put the phone to sleep and let it sit and it seemed to sort itself out. Both instances only occurred after the Windows Feedback app was opened for the first time. It was a very strange bug and ruined Build 10572 for me (although I could tell how much of an improvement it was in other ways.)
Upgrading to Build 10581 was easy and joyful. I started the upgrade and it completed about an hour later with no reboots or errors (I have gotten errors in the past, but only when upgrading from one Insider Build to another.) Once it was done I decided to do a hard reset, even though I believed it probably wouldn't be necessary. I was trying to avoid any lingering affects from that crippling phantom-touch bug and to ensure that this Build was, in fact, the smoothest yet.
The hard reset went flawlessly and I went about setting things up. Really this just means moving a couple Tiles around the Start Screen, changing the wallpaper and Lock Screen to the Windows 10 "Hero" wallpaper, and changing the accent color to grey (I've grown fond of it with the Windows 10 "Hero" wallpaper,) also slightly increasing Live Tile transparency. I don't feel like setting up the entire phone to my preferences when I'm likely to wipe it all anyways with the next build, and the default Start Screen is actually quite good (I'm happy they moved Settings to the folder in the lower left-hand corner and Excel to the little group of Office apps, like I kept doing myself. It just fits a little nicer.) I'm sure you can see the "improvements" I made which really meant removing Facebook and Skype from my Start Screen and resizing some Tiles to make sure everything fit.
The first thing I noticed is just how fast everything was. In 10572 the biggest, most immediate indication of performance improvements was actually the speed of the context menus, which were nearly instantaneous. In 10585 they're marginally faster. Switching between apps is more stable (even with only 512MB of RAM), and apps crash way less often (although the Store still crashed once or twice while updating a bunch of apps.) I had a couple freezes and such, and I had two black screens going to the Start Screen, both from the Photos app and one resulting in a ...Loading... screen.
Otherwise performance is exemplary and I'm very pleased. On my Lumia 520 at least it is actually a little more speedier than Windows Phone 8.1, if not as stable. Navigation is a little nippier but it still doesn't feel as polished as Windows Phone 8.1 did. For example, I had one instance of Tiles flickering on the Start Screen (still far better than earlier builds.)
It's too early to tell with battery life but 10572 was actually an improvement (the battery is starting to fail as a whole so it's not that great anyways, I could get a good day out of 10572.) I didn't notice any changes although it's not been very long. It seems that Insider Hub has been removed, which is unfortunate (strange how they keep removing it.)
When I went to Settings, the normally over-packed Extras was abnormally empty. I'll make a conjecture and say that either Microsoft is stripping away the no longer necessary Nokia guff or it has something to do with the Store updates.
However, I carefully monitored my app updates and I only had one error, and that was for access point. After restarting the update it updated flawlessly and gave me no more trouble. I had 33 app updates right off the bat but I knocked that down to 21 after I got rid of all the pre-installed apps and uninstalled them. Many of these updates were nearly instant, and I suspect they were only to set those apps up to work on the new Build (like Calculator, which would open but wouldn't work until it updated in the Store.) When attempting to share the above screenshots through an email to myself (no way I'm waiting for my OneDrive to sync up then mess around with downloading all the pictures through the browser) I had a strange instance of the Outlook application setting up and beginning to start than freezing on the splash screen.
However, on the second try it worked perfectly fine.
All in all this is definitely the best build I've had yet and I put it above Windows Phone 8.1 in performance in both battery life (making this assumption based off 10572 performance and battery life improvement claims) and general navigation, but below in stability and polish. There are still some UI and UX improvements I wish to see and it's still very rough around the edges but in many ways this is shaping up to be the best iteration of Windows Phone ever. Happy I got this installed as soon as possible, as it's given some new life to my Lumia 520. Someone who knows nothing of phones could never tell that this phone is using an outdated, extremely low-powered processor based on operating system performance alone, and that's exactly what we want. Already better than Android, clearly. I'm happy at the speed at which Microsoft has improved Windows 10 Mobile (two builds have both seen drastic improvements) and I hope they keep up the good work.
Have a lovely night everyone.
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