Unusually, a build of the main branch of Windows 10 Mobile has crept into production status without going through the 'Release Preview' Insiders ring first. Possibly because of time constraints, possibly Microsoft was just extremely confident in this one. Certainly the previous RP build (242) was very stable for me on the Lumia 950, I'd only had to restart the phone twice in a week (which is good for Windows 10!)

From the support forum page for the May cumulative update:

We have started rolling out the May cumulative update (10.0.10586.318) for Windows 10 Mobile today. 

Key improvements in this release include:

• Reliability, performance and stability improvements.
• Fix for a problem that could result in unexpected battery drain while the phone screen is off. 
• Improvements to the update system to prevent phones getting into a state where attempting to take an update may lead to errors 0x800f081f or 0x80070570. 
• Fix for a problem that caused some phones to finish the initial setup wizard with an incomplete set of app tiles on the Start screen.
• Improvements to the reliability of USB-C connections. 
• Improvements to Cortana, including a fix for a problem where music playback would not resume after dismissing an incoming text, and a problem with Quiet Hours not setting automatic rules for every day. 
• Fix for a problem that could result in the phone to hang or restart when visiting certain web pages in Microsoft Edge. 
• Fix for a problem that caused recorded video to be lost when answering an incoming call on some phones. 
• Improvements for reliability of internet connection sharing (ICS) and tethering.
• Fix for a problem that caused the navigation bar to overlay and hide part of the screen for some apps on certain phones.

Good to see, anyway. Our editorial devices are updating at the moment and I'll comment back later if there's anything notable to say. For completeness, note that anyone on the Threshold 'Release Preview' Insiders ring also gets this update, i.e. it's available across the board, for production users and Insider alike. (Note that all this is separate to the next-gen Redstone Insiders branch and rings. See here if you're confused!)

This is also what's currently available to all those with non-approved W10M devices, e.g. the Lumia 1020, 920, 925, and so on, via the Insiders programme, so it's notable for them too. Comments welcome on how this latest build performs on the older hardware.

For production 950, 650, and even officially upgraded devices, this is just another stability update, of course.