From the Microsoft announcement post: (and with my emphases)
Today we are rolling out Windows 10 Mobile Insider Preview Build 10581 to Windows Insiders on the Fast ring. We’ve fixed the bug preventing upgrades from earlier builds. So if you stayed on Build 10536 or if you flashed back to Windows Phone 8.1 and then installed 10549 or 10572 you can upgrade directly to this new build without going back to 8.1 again. This build doesn’t have significant new features but does include a bunch of good fixes and general performance improvements.
Here are some things we have fixed:
- For devices upgraded from Windows Phone 8.1 like the Lumia 930, Lumia Icon, and Lumia 1520 that had the “Hey Cortana” feature – with this build “Hey Cortana” should work again in Windows 10. To make sure it’s turned on, just go to Settings > Extras > Hey Cortana.
- You can now select photos for sharing through third party apps like Facebook, Facebook Messenger, Instagram Beta, WhatsApp etc.
- Battery life should be improved in this build. We fixed several bugs impacting battery life.
- We fixed an issue where you couldn’t choose some apps to show quick status on the Lock screen via Settings > Personalization > Lock screen.
- We’ve made improvements to text prediction and auto-correction.
- Video recording functionality has been improved.
- Visual Voicemail sync should now work properly.
- We fixed the dual-SIM issues that appeared in Build 10572.
Our test devices are flashing the new firmware as I write this, but your comments are welcome once you're on board too. You probably saw my article talking about even the mighty Lumia 1020 now being practical under Windows 10 Mobile, expect more features in the same vein but referencing 10581 directly and for other devices, price points and use cases.
Initial observations on 10581 are that it is indeed slicker (I haven't had ANY 'resuming' when going back to the Start screen so far - yay!) - and, for once, (at least coming from 10572) there's not a barrage of Store application updates arriving in tandem. All that appeared was an update to Windows Camera, to be used by every device other than the 1020.
In terms of bugs or issues, I've noticed that Storage Sense seems to be broken. So look out for a fix for that arriving via the Store.
Microsoft's drop dead date for finishing firmware for the new Lumia 950 and 950 XL is in about three week's time - I think they'll make it. Do you agree?