We have this same discussion about once a month or so. Search through the archives using the search engine and they should show up. Set the "newer than" box to 1 year and one such discussion will show up for sure.

I have a Bibler Winter Bivy, but that's just to keep my bag dry. It just looks like a sleeping bag cover with a zipper.

The fancier bivys, to me, are more for climbers who don't want the high profile of a tent in nasty conditions. Other than that, I think they have been superceded by the new lightweight tents, such as Henry's Tarptents, SMD and a couple of others.

I would much rather be in the tiniest tent than a bivy, especially in bad weather.