Numerous variants of the Galaxy S5 and Galaxy S6 family have been upgraded to Android 6.0 in many parts of the world, as has the Galaxy Note 5, most recently in India. In the US, the Note 5 has now started to receive the update on Sprint too.
As SamMobile reports, the update weighs in at 1362MB, and along with Google's own improvements that it baked into Marshmallow, it also features some of Samsung's own additions, including its TouchWiz interface.
As the upgrade's release notes point out, some apps installed with the previous Android release, 5.1 Lollipop, may need to be updated after the Marshmallow installation is complete.
Update: As GSM Arena notes, the update is now rolling out to the Galaxy S6 edge+ on Sprint too.