Well that was fast. Only a few days after the Insiders Preview enthusiasts got to test build 10586.29, it's now available to release hardware for regular users, in this case for the Lumia 950 and 950 XL. The speed of the update is notable for a couple of reasons.

If you have one of the new devices, you'll find the update in Settings / 'Update & Security' / 'Phone update':

That this update was made available so quickly indicates that there are some important under the hood fixes. The 950 and 950 XL have been pretty stable for me, so I can only guess at power drain issues being addressed - this certainly comes under the category of 'critical' and there have been some (but not universal) complaints about poor battery life.

Plus we know from the Insider build that the followinf feedback was addressed for .29:

• Improved application backwards compatibility for Windows Phone 8.1 Silverlight applications.
• Edge performance and stability has been improved.  Auto-completion has been updated to allow user to more easily edit the end of the URL in the address bar.
• Additional Bluetooth stability improvements.
• We have resolve issues on switching the active cellular connectivity profile on Dual SIM devices.
• Data profiles are now correctly restored in the out of box experience that would prevent some devices from sending or receiving MMS messages.

In addition, Terry Myerson at Microsoft tweeted: "Big day, first update worldwide (and more to come) to all phones running #Windows10 - same build as PC!" There's an implication here that the phone/mobile builds of Windows 10 will be kept more in lock step with the desktop builds from now on. Given how far Windows 10 Mobile had to come to catch up, this is extremely good news. More updates, better software, more often, and so on.