T-Mobile have started seeding the OTA for Samsung Galaxy Note 5 users in US. All other carriers have sent out the OTAs and T-Mobile becomes the last one to do so.
The OTA will have usual Android 6.0 Marshmallow features like doze mode, app permissions, custom Google tabs etc. Along with this, T-Mobile has decided to incorporate a few more features. The Marshmallow update will have a Samsung browser with ad-blocking capabilities. Some users have also reported a faster Note 5 after the update.
Also, some of the bloat apps which were present in the device will be deleted. But only those incompatible with Marshmallow are removed while many of them still remain.
The OTA update is almost 1.4GB in size. It will be rolled out in phases so not all the users will receive simultaneously. If you want to check whether you have received the update, go to Settings >> About Phone >> System Updates.