Only a minor update to the existing Windows Insiders build (from .11 to .29) but a new version of the OS is always welcome, so get updating/checking all your Insiders devices now. The main improvements seem to be around helping out in the big 8.1 to 10 transition, a process which does seem to be getting smoother all the time, though I'll admit that I'm usually going from a fairly recently reset device. This upgrade needs to work, in a few weeks time, for real world users and all their data and apps. Gulp.

From the Microsoft announcement post:

We have received a lot of useful feedback from our Insiders on our last Windows 10 Mobile Insider Preview build 10586. While this is the same base build that we are shipping with new Windows 10 devices, we are continuing to address feedback on the upgrade experience from Windows Phone 8.1 as reported from Windows Insiders.

Today we are releasing our first Cumulative Update to the Windows 10 Mobile Insider Preview Build 10586 to Insiders that begins addressing this feedback. The first thing you will notice with this Cumulative Update is it will use the minor version for differentiating between each version as these builds only contain fixes to the Windows 10 Mobile Insider Preview build already in the Fast and Slow rings. So this build will appear as 10586.29.

Feedback we have addressed includes:

• You will see additional improvements in upgrade experience, including devices with limited free space, map rendering on update, and RCS enabled device settings.
• 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.

As usual, the update takes about half an hour to apply, so allow for this. I doubt you'll notice much difference in day to day use, but comments welcome as usual.

Note that if you don't see this update (10586.29) then you probably need to go back into the 'Windows Insiders' application and approve access to the 'fast ring' again. This may involve restarting the phone.