I carry this only as an emergency bivy sack. I've got a couple other bivy's that are designed for regular use, but since this sack is the size of my fist it goes with me on all my hikes, snowshoe trips, x-country ski day trips, paddling trips, and off-road adventures. I figure if I ever need to spend an unplanned night or two out I'd rather do it without shivering all night. If you are looking for a bivy that breaths good (low condensation) you need to look at Goretex or E-vent or something that is not as quite as compact or inexpensive as this bivy. But if you are looking for surviving a couple nights until you find your way out of your predicament, this is probably what you want.
john m I was worried that if I ever used this in really cold weather that it would hold in the moisture and collect the condensation just like a tent does in the winter. In a winter survival situation where condensation is a problem I suppose it would be possible to open it up every few hours to let out excess moisture. That is a good point about the weight difference between this Bivvy and the heavier solutions. Thank you.