For any of our Android developers out there, it's already possible to get hold of the Android 6.0 Marshmallow Developer SDK. Even if you aren't a developer, you are welcome to download it and play around with it. Here's the link where you can grab it: Downloads Android Developers
For now the only compatible devices you can test with are the Nexus 5, Nexus 6, Nexus 9 and Nexus Player. If you are ready, willing and able, the instructions for flashing the new version can be found here: Factory Images for Nexus Devices Nexus Files for Developers Google Developers
If any of you folks give it a go, please shout out in this thread and let us know how you like it!