Cyanogen today announced the CM 13 (Android 6.0 Marshmallow) Nightly builds for the first set of devices, The devices are
OnePlus One (bacon)
Nexus 7 (deb & flo)
LG G4 (h815)
Galaxy Tab Pro 8.4 (mondrianwifi)
LG GPad 7.0 (v400 & v410)
LG G3 Verizon (vs985)
Moto X 2014 (victara)
As a OnePlus One user, I have received the update.
There are some warning for the users who are running the CM 12.1 stable version. Cyanogen says you will feel a dip in quality if you are using CM 12.1 YOG4P or CM 12.1 YOG7D and says quality approved version of CM 13 will be released by Jan 2016. If you are using 12.1 nightly you can flash the new version and you will have to upgrade your apps like Xposed, OpenGapps, etc.
If you are on a 12.1 nightly, you can ‘dirty flash’ CM13 (read update like any sane person) as long as you also update your third party addons in the same pass. This means, if you use Xposed, OpenGapps, etc. flash a Marshmallow compatible version of them immediately after your CM13 flash.
Just a note for those who tried the experimental builds
CM 13 Experimental is different from CM 13 Nightly, Experimental was a test run for the build bots.