Sprint customers have a lot to deal with, starting with the fact that they're paying for Sprint service. (I kid, I kid.) But those of them who use a Galaxy S6 or the curvier Galaxy S6 Edge have reason to celebrate, as both phones are being upgraded to Android 6.0 starting today. We have confirmation from multiple users that the OTA files are going out, and Sprint's support pages for both phones say the same thing.
Sprint says there's nothing new in the software aside from the AOSP update, which seems a little simplistic considering the 1.2GB update size. The changelog that accompanies the update notification on phones makes mention of adjustments to TouchWiz and a new vibration pattern, plus the rest of the bells and whistles that come along with Marshmallow. It also says that device encryption can't be disabled after updating the software, so decrypt your phone beforehand if you want the option.
As usual, rooted or otherwise modified phones won't be able to apply the vanilla update, and it may be a week or more before the rollout makes it to everyone. (If you're desperate, check XDA or other user forums - for popular phones like the S6 and S6 Edge, OTA files are often available for manual flashing.) Get after it.