True to its words, Google started rolling out the Android 6.0 Marshmallow update for its Nexus lineup of devices earlier today. For now, the OTA update seems to be rolling out only for the Nexus 7 and Nexus 9, but it is likely that other Nexus devices will be getting it sometime later today as well.
Android 6.0 improves a number of core features of the OS, including improving battery life thanks to its new power saving optimisations like Doze and App Standby; an App Permissions manager, and native fingerprint scanner support.
In addition to the factory images, the company has also pushed the Marshmallow factory images on its developer website. So, if you cannot wait for the OTA update to be available for your device, you can always go ahead and flash the update manually on your device.