Excellent question! I did not mention any names because my period of extensive bivvy use is - uh- "historical" and the types I used are no longer available, especially a double bivvy, which I don't find among current offerings.

A bivvy is not especially complicated. In the early 70s, I ordered a couple of yards of new-fangled Goretex, some coated nylon and simply stitched them together and coated the seams. It worked wonderfully - the first time I used it, I awoke in the Grand Canyon with a six inch puddle atop the bivvy - inside I was wonderfully dry and the loft of my bag was unimpaired.

The double bivvy worked great for SAR operations. We were always partnered up and the bivvy was a key ingredient that enabled us to stay in the search area overnight comfortably. On one occasion we stopped about a half mile from our victim - finding here promptly the next morning after about twenty minutes walking. I often thought that if we had walked that extra half mile and found her, we could have shoehorned three bods into the bivvy and passed the night - tight, but warm, with the outer individuals in sleeping bags and the person in the middle wearing everything available.

For current brands, with which I have no actual field experience, i would look at REI's minimalist bivvy, weighing about a pound and MSR's similar product which appears to be about the same, only twice as expensive. Both feature breathable tops, with coated bottoms.

What puzzles me are products like Outdoor Research's "Advanced Bivvy" which is roomier, but costs over $300, and weighs two and one half pounds. For the same weight, and less money, you can carry a real solo tent, which will give you more room.

Basically, today you can choose a point on a continuum running from AMK's light and cheap (but clammy!) bivvys on up to glorified tents - whatever suits your fancy.

In my old age, I don't seem to utilize a bivvy as much as I did in my youth. But I still carry AMK products - they are light and serviceable. They can come in handy in providing protection for a victim or work as a body bag, if necessary, in addition to providing overnight shelter.

The bottom line is there is no quicker, lighter way to shield your tender bod from the elements, in a variety of situations, than a bivvy sack.