Here's the quoted changelog, some of which refers to Windows 10 Mobile on the desktop, so take these items with a pinch of salt:
- Improved reliability of Windows Shell, map apps, Internet Explorer 11, and Microsoft Edge.
- Addressed issue causing push-button reset to not work correctly and roll back on devices with language set to any of the Unicode languages.
- Addressed issue causing devices to crash after disconnecting an e-reader that was attached to the device.
- Addressed issue causing devices to not recognize a Secure Digital (SD) card if it is inserted and removed multiple times.
- Addressed issue causing some apps to not respond to the commands in the app bar in Windows 10 Mobile.
- Addressed issue that sometimes blocks alarm notifications in Windows 10 Mobile.
- Improved support for using the camera app in Windows 10 Mobile Enterprise.
- Addressed additional issues with rendering 4K resolution, missing Start menu tiles when running on battery, Internet Explorer 11, Microsoft Edge, Bluetooth compatibility, graphics, display rotation, app compatibility, Wi-Fi, Feedback Hub, Miracast, Windows Shell, revised daylight saving time, and USB.
- Security updates to Microsoft Edge, Internet Explorer 11, Microsoft Graphics Component, Windows kernel, and Adobe Flash Player.
As usual, these Insiders builds are available to anyone on the Insiders programme, and with a compatible device (think Lumias 550, 650, 950 or those upgraded officially, according to my original chart). The Release Preview ring is definitely the one to be on, since the Fast and Slow* 'rings' are (or are going to be) deep in early-beta 'Redstone 2' land for months.
* In fact, the 'Slow' ring Insiders also seem to be being pushed this build, but this is a temporary situation - as soon as Redstone 2 is stable enough, 'Slow' ring Insiders will be automatically bumped up. So being on the Slow ring now is a bit of a 'Do I feel lucky' situation!