I think the bivy so far is the best I've found so far for protecting down. I'm experimenting with a OR MicroNight Bivy and I'm thinking it's not completely waterproof from the comments from many people from different forums, The consensus says that it can take some light water but not a full storm and recommend a tarp or poncho over top. It's a very compact and light bivy and it definitely makes the temp rating increase (probably by 10 degrees) but I haven't field tested it yet but I look forward to see how it handles in a hard cold rainstorm.

http://www.outdoorresearch.com/site/micronight_bivy.html

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Failure is not an option!
USMC Jungle Environmental Survival Training PI 1985