As I sit in New England the temp tonight is hovering at 0! I think of the most extreme times I've used my complete military sleep system and am greatful for the military creating the new system. One night on the ground in Maine in mid December. With the complete system and poly pro underwear I was as warm as could be! The other is Fort Drum NY in Januay prior to leaving for Iraq (-40!!). In a tent (no stove) on a cot with the comlpete system and poly pros and still as warm as could be!
The only draw back I've ever felt the system has is that there is no mesh screen in the head area of the bivvy sack. I've spent nights out in Maine during black fly season with my civilian bivvy sack (screen) with no problem what so ever. My Father did not have one and had one horrendous night! I've never tried it (black fly season) with my mil bivvy but black flies work their way into any opening they can so I think the results would be bad.
Other than that one concern, I'm with you, they are a great addition to pick up.

Just received 2 surplus Marine Corps goretex woodland camo bivy covers. These bivy covers are rain/snow waterproof goretex covers that slip over a mummy bag to protect you from the wet weather. They are both in excellent condition and look brand new. Can't beat the price at $40 per unit, they are $140 for a new one if you can find them. These units fold up into a small package and are lightweight for easy carrying and make a nice addition to your bug out bags sleeping equipment.