I actually bought a North Face bivy from Chas and so far it's great. While my buddies have lugged around a monstrous Bibler bivy sack, I have been carrying this small, packable, lightweight jobber. A bivy sack should be just that....a sack for minimal bivying. If both you and your partner carry a bulky well ventilated bivy sack, then you may as well split a small tent, eh?