I went out when it was 2 degrees outside, and it wasnt that bad, there was a wind so with windchill it was about zero. Yes i did have lots of layers on and sweats under my drenalin pants, and I was warm. I was really impressed with my UA Browtine boots my feet did get cold but they got cold around noon. I figured that wasnt bad for as cold as it was and they only being 800g. I think I would still go out if it was in the negative teens. I love being out there and you cant get any deer sitting on the couch wishing it would warm up!!
If I lived in an area that had big bucks, I might consider hunting in the bitter cold. As it is...when its below about 15, I probably ain't going. There's a 99% chance I'm not gonna see a big one anyhow, so I might as well not freeze. Once in a while I'll hunt in the real cold, but it ain't often....and I almost never do outside of the rut.
It's never been too cold for me. I may not hunt all day but I'm out for a few hours. "Live Life at Full Draw"