This is just the first phase of the Marshmallow launch. System images are a manual, immediate way to update, but an OTA update should be rolling out to more casual users soon, which will do an in-place upgrade. Google should also be updating the public AOSP codebase with the final Marshmallow code today. If you're looking for fresh hardware to go with your new operating system, the Marshmallow-packing Nexus 5X and 6P are shipping sometime this month.
The final build of 6.0 is "MRA58K." We've taken a peek at the build and found no major differences from the Android M Developer Preview #3 that has been out almost for a month and a half—it's just bugfixes. This build also counts as the Nexus program's monthly security patch for October. There should be a security bulletin posted on this page soon.
Android 6.0 Marshmallow brings user-controllable permissions, automatic app data backup, a fingerprint API, and a ton more features to Android. If you're interested in more details, we have a massive, 15,000 word review right here.
