Microsoft is pushing forwards at top speed for Windows 10 Mobile now, with a major new build for the Insiders community. See below for the official release comments. The focus in build 10512 (over the previous 10166, a month ago) is in the new core models and a long list of critical bug fixes. And yes, this build really is worthy of use day to day, in my opinion.

From the Microsoft launch article: (with my emphases)

We’re rolling out Build 10512 of the Windows 10 Mobile Insider Preview right now to Windows Insiders on the Fast ring. Thanks for your patience waiting for this one, the team was very focused on our Windows 10 release for PCs and tablets, and we needed to do some prep work for Mobile to move to a new branch. I expect the builds to be out more frequently again as we go forward from here. Our major focus on Windows 10 Mobile right now is on improvements to core quality.

We’ve made quite a few improvements since the last build, but here are a few things that we fixed that were frequently reported by Insiders:

• General improvement in overall stability and performance.
• You can now set the Lock screen and background image from within the Photos app.
• We fixed an issue where apps installed on SD cards weren’t working after a reboot.
• We have improved Latvian and Chinese inputs.
• We have made improvements to the Kid’s Corner tile layout.
• We have improved reliability of Data Sense notifications.
• Modified Shape Writing algorithm to be less aggressive in suggesting contact names over common words.
• Resolved an issue where Camera would not launch when the phone was locked.
• We fixed an issue where notifications for incoming text messages weren’t appearing.
• We fixed an issue where the touch screen wouldn’t be responsive after ending a phone call on some devices.
• And we fixed an issue where folder tiles have obviously overlapping text.

The sheer version/build jump should indicate the scale of how much has been changed and tested under the hood though, well over 300 internal builds of various magnitudes (though, obviously, most didn't make it very far within Microsoft's teams).

If you don't see the update and you're on the programme, try going back into the Windows Insiders application (which you may need to install again) and going through its set-up sequence again.

As usual, there's the dilemma as to how to handle the update, for those already on the Insiders programme. For previous builds the thinking was that a factory reset after the update, following by rebuilding things manually, with data syncs and Store installs, was the way to go to minimise glitches. And I write that from bitter experience from four devices now on the Insiders programme.

However, with bugs being quoshed right, left and centre, I think it's time to backup using the built-in Windows 10 mechanisms (see Settings/Updates and Security), just in case, and then just let the over the air update take place.

If all is well then you're good to go, if it's not then factory reset and then restore from your backup. Either way, you shouldn't have to rebuild everything from scratch again. Your comments and data points welcome! In my case, the editorial Lumia 1520 updated 'over the top' just fine and everything's working perfectly. (Well, apart from the things that the Windows 10 Mobile team hasn't coded yet. Like Glance screen settings...!)

[Of course, if you're new to the Insiders programme or are coming from something other than build 10166, then you should definitely go down the factory reset and build from scratch route. Also, note that on most devices, you'll probably be taken through multiple update steps, i.e. you'll have to go 'via' build 10166. So allow plenty of time and re-install the Windows Insiders application if needed. Don't worry, I'll have a full tutorial very soon here on AAWP.]

As usual, after the update, your first trip into the Store may result in a bit of a wait while some of the main applications that aren't part of the actual firmware anymore also update themselves. Please do allow for this. In practice, there were only a handful, though did include the Store itself:

'Block and Filter' is new to me, but seems to be an under-the-hood security module, not showing up in the main application list.

So by all means do that backup first, but I was very pleasantly surprised that everything's working fine without any resetting this time around.

It should also be emphasised that the speed of grabbing this update and the Store may well be limited by the number of servers Microsoft has got allocated to Insiders. The bottlenecks I'm seeing look more server-side - obviously the vast Microsoft server farms will be made available when Windows 10 starts rolling out for all form factors through the rest of the year.

I'll update this post with any major changes and improvements that warrant detailed coverage, during the evening. There are some known issues noted by Microsoft: (again, with my emphases)

• Mobile hotspot doesn’t work in this build. When sharing the Internet from your phone using Mobile Hotspot, devices will fail to get a valid IP address from your phone and will show no internet access.
• Two-factor authentication for your MSA with a phone number doesn’t work in this build. If you reset your phone and go through the first run experience, you won’t be able to input more than 2 characters for validating phone or text authentication. If you have a secondary email for two-factor authentication – you can choose that. As a workaround, add secondary email for two-factor authentication.
• Some phones might spend a long time in the post update migration experience – just let the phone sit and it should complete this migration.
• Apps that use background audio such as Groove, Podcast, Pandora, etc. will crash when launching after receiving an app update from the Store. Rebooting the device will resolve this issue.
• Some phones with a large number of tiles pinned to the Start screen may get stuck in a state where the device shows “Loading…”; the device will need to be reset or rolled back to Windows Phone 8.1 using Windows Phone Recovery Tool.
• Some apps may fail to update through the Store with the error code 0x80073cf9; uninstalling the application and reinstalling it will resolve this issue.
• The Movies & TV app will not work in this build (video playback is currently broken).

While you're waiting and downloading, make sure you've read my 'Windows 10 Mobile a step-change, but ultimately needed on many levels', plus my recent benchmarking features.

The two factor bug is unfortunate. I've just disabled this from my microsoft.com account settings panes (on the Web) and have set a diary reminder to re-enable this in a few weeks time. Or you can opt to go just with email verification if you're particularly paranoid(!)