My son and I have HBS and love them... we have no problem doing all day sits when it's around 0 degrees out in the late season.
I wish the legs weren't separate, but that's my only complaint. I started dressing lighter and lighter and using my HBS more and more. Even when it's around 45 degrees or so. I'll go out with just a tshirt and a hoodie and I'm fine, and don't break a sweat walking in.
It also conceals a LOT of motion when you're using calls, etc and any little movements you might make if the deer is behind you.
I'm starting to think that it traps a lot of scent inside of it too, but don't have any proof.
It's saved me TONS of money on trying to layer up and buying heavy winter parkas, etc.