US Modular Goretex Sleep Bag System MSS Complete 4 part (Rated -40F)
Product Review (submitted on August 12, 2007):
One thing to understand is that, 99% of the time, you're really not intended to use the thin or thick bag by themselves - they're meant to be covered with the bivvy cover, to protect the bag from wind, dirt, moisture, etc. The bivvy cover is waterproof/water resisitant, and made of a material that you can really just brush dirt off of, whereas the bags themselves will sponge up muddy water, wind will cut through them, and dirt will be a pain to get off.
It also makes it a few degrees warmer. Just my 2 cents.