Here's what other sites are saying:
A good night's sleep starts with the proper shelter, such as the Ultralight Bivy Sack from Mammut. The top of the bag is Mammut's ShelterTX fabric and the bottom is made of a slightly more durable EnduranceTX; both are very waterproof and utilize a polyurethane membrane to prevent moisture from getting at you. An integrated stuff bag makes this bivy extremely easy to bring with you, either on your travels or on the trail.
The Mammut Ultralight Bivi is an uncompromising and very lightweight bivouac bag that should be an essential piece of equipment on any tour. It is equipped with rope openings, a draw cord and integrated stuff bag.
Black Diamond Big Wall Hooped Bivy
$250 - $269
Outdoor Research Advanced Bivy
$60 - $320
Big Agnes Three Wire Bivy
Sierra Designs Backcountry Bivy
$130 - $149
Kelty Classic Bivy
$63 - $89
The North Face Assault Bivy
$68 - $99
$139 - $205