Android 5.1 has been released for several Nexus devices including the Nexus 5, 10, and 7-WiFi. You can expect it will probably be rolling out in one form or another to the rest of the Google line of devices in the near future.
Android 5.1 is still codenamed Lollipop, boasts enhanced stability and performance, and offers new features such as native multiple SIM support in devices that contain multiple SIMs (no more aftermarket software), and 5.1 Device Protection, which is something I’ve wondered why it wasn’t around for a long time.
With Android Device Protection, if you lose your phone or it gets stolen from you, it’s useless to anyone else for anything other than parts. Factory resetting won’t make it usable to a thief. This is something that really should have been done years ago as device snatching pays thieves enough to make it desirable.
Along with the above, Android 5.1 introduces a high definition voice calling feature on compatible networks for those that dare to use a phone as a phone. My carrier’s been advertising it’s coming for about two and a half years and still no go, so it’s nice to see Google pressing the issue.
There are also some enhancements to allow direct connection to BT and WiFi devices from the Quick Settings Menu as opposed to opening a sub-menu.
Android 5.1 was being uploaded to the AOSP 15 hours ago and should be in the hands of custom ROM developers reasonably soon, if not already.