I sit in sub zero weather every year and the majority of my sits are below 40 degrees for the entire year, maybe 30-40 sits with colder temps. I am never in the woods, even during the warm weather 50 and 60 degree days, without my muff. I have 2 muffs, one with windstopper material, the other without. I clip my range finder to it, and use it to carry my gloves, facemask, hat, grunt tube, etc. If it is really cold, say in the 20's or lower, I wear a heavy pair of gloves to walk into the stand in, then place those gloves inside my jacket next to my sides to keep them warm, as well as add extra warmth to myself. I then slip on only a thin pair of jersey-weight thermax or polypropolene type gloves. Those type of gloves are all I ever wear when hunting, even in below zero temps. If it's in the low 20's or colder for the entire sit, I will use up to 2 large heat packs to keep me comfortable. If it is raining or snowing hard, I will use a saddle-cloth gore-tex gator I bought specifically to wrap around the hand-muff. I have no idea why they havn't come out with a gore-tex insulated model, I even called Cabela's product development department and suggested it.
I've been hunting with a muff for 17 years now, ever since I made my own first one with several layers of wool and a large safety pin when I was 16. Bow, arrows, release, safety belt, hand muff. That's the order of importance I place on it as part of a mental check-off before I enter the woods, any time I hunt. Give me warm clothes, my mickey mouse boots, hand muff, neck warmer, knit facemask, and a gore-tex knit hat, and I've logged many hours of below 10 degrees weather.