Let me start by saying I wish I had this piece of kit when I was in the military.
This system has 3 main components.
This is the first part of the system that I bought a few years ago. It is water proof and really helps to keep you bag clean. It is also big enough to fit extra gear with you inside it. It has a very solid zipper and a flap that snaps over the zipper to keep the weather out.
Intermediate Black Bag
For me this is about a 20 degree bag. It also has a great zipper. There are baffles on the chest and zipper. This really helps to keep the heat in. One of my favorite features is the double insulated foot area. The bag is tapered and has a mummy style hood with draw string.
Green Patrol Bag
This is my least favorite part of the system. For me this is a 50 degree bag. It does have the same beefy zipper. When used with the Bivy it is good for Summer tarp outings.
When layered they make a great winter sleep system. There is an order in which to put them together.Gortex Bivy is on the outside, next is the over sized Patrol Bag, and the Intermediate Black Bag is the inside layer. If you line up the zippers it is easy to snap it all together.
I have used this system in below freezing temps and never been cold. You can get them from 100-250 depending on condition. in my opinion,for the money you really cant beat it.