I received the Integral designs south col bivy for christmas and used it 5 nights for short (1 or 2) nights winter trips and I have to say that this bivy bag really impressed me during my last trip.
The weather was pretty cold (btw -5 and -10F) and humid. It was really unpleasant. I sweat a little bit in the my sleeping bag. And in the morning there where no condensation at all except on the sewing. The bag 2 breathes pretty well and I was really happy with it.