Never used one. Must be missing out on the good life.
Don't think I missed a day of huntin during the last three seasons too. If I did, the weather must have been really shitty.
It's the wind that usually does me in and will typically cut my hunts short, especially when it's 20 degrees or less with above 10 mph winds. Also, when is that cold out, I try to sit, either in one of my tree-stands or blind, the first couple of hours of daylight, or however long I can possibly stand it, then go do a still hunt afterwards to get my blood flowing again, which warms me right back up on those really chilly days and the wind has to be in my face, or I'll head back to the shop and get warmed back up by the cherry hot woodstove.
I looked into heaters for the blind I think last year, but never pulled the trigger on getting one yet. I still might so, I'll be paying close attention to this thread, thanks!