I was planning to avoid updating to 6.0.1 for as long as possible, but then the software update notification panel started showing up all the time, so I took the dive and updated to 6.0.1 rather than continually be annoyed by the software update notification. My initial impression is that except for some redesigned icons, Marshmallow is exactly the same as Lollipop from a day-to-day usage perspective. Everything seems to be working normally, and the battery consumption rate is not out of the ordinary.
After a few more days of using Marshmallow, my opinion is still that its effectively the same as Lollipop from a day-to-day usage perspective. My wife and I both have GN4's, and we've had none of the issues that others have mentioned here, and if anything battery life and speed are both a little better.