Warning for GN4 users, based on content of message boards the GN4 Marshmallow 601 update is doing exactly the same thing that the Marshmallow 60 update to the GS6 did.
The loss of functionality from the previous OS was an issue as well but completely over shadowed by power issues.
After update to my GS6 the phone would endlessly trap in a loop of starting and crashing.
Reinitialising the phone did not correct, all it did was lose data and apps that were loaded.
Holding Vol UP Home and Power to access root boot up screen and clearing caches seemed to stabilise for a while but no joy in the log run.
Still almost daily I have to hold Vol Down and Power buttons to force clean re boot.
Battery would overheat and loop into start crash randomly. Clean boot would cure for short time.
Only way to charge the phone was to close all open apps and wireless or usb charge.
Rendomly would still trap in restart loop while charging
OR
Whilst on charge it would charge to 100% then discharge from full to 0% in 2 hours and then recharge until it repeated or went into trap of shut down reboot cycle.
Many GS6 users have reported exactly the same issues and there is no joy from any source.
Marshmallow 601 update released short time ago, and was applied automatically, with no change to the manic nature of the phone.
But Hooray the GS6 phone is now a brick and has been certified as such, warranty replacement GS6 is on the way.
I am not even going to charge it I will pay the fees and upgrade to the GS7 which comes with Marshmallow and block all upgrades.
This is for your edification and enlightenment.
I know 99.9$ of people will not check the bulletin boards before upgrading but spread the word PLEASE.
MARSHMALLOW has serious issues as an upgrade.