Hi de hi campers!
Having recently purchased one of these bivvi bags, I thought I might offer an updated opinion over that which was submitted by the OP some years ago. I thought it was worth updating this threat because there doesn't seem to be an awful lot of information doing the rounds on this particular product.
I bought the Mil-tec 3-layer waterproof bivvi bag (Coyote Brown version) back in November, where I found it was on offer for £36 delivered (my google-fu skills suggest you can currently get it from various outlets for £49.95). If I'm honest, I wasn't sure about it, but I fancied taking a punt and didn't stand to lose too much.
It is, as far as I can tell, a copy of the sought after USGI goretex bivvi bag. The 3-layer fabric certainly feels very similar to my British Army goretex bag, but my mil-tec bivvi also had a very robust almost rubberised base to it. Having used it a couple of times now, I've been very impressed with the overall build quality of the product as well as it's robust fabrics.
Two downsides to note though are that those robust fabrics mean that it is rather heavy and a little bulky. By way of comparison, I'd suggest that it's marginally heavier and bulkier than the British Army bivvi.
In use, there's a lot to like about this bivvi. On it's first outing, I was under a tarp, but had to endure cold temperatures, very strong winds and heavy rain which was being blown under my tarp. The bivvi kept me and my sleeping kit lovely and dry and it also proved to be very effective at keeping the wind off of me. I also noticed how spacious it was. I'm 5'10", a rather rotund 16st and had ample room in the bivvi for me and my Exped Downmat. The other thing that I really liked was the hooded section as I found I could pack my spare clothes into this section and create a really comfy pillow which is held nicely in place by the hood.
For the most part, I slept with the zipper fairly open and my head uncovered. I did however have to hunker down when the heavy rain set-in. No issues with feeling claustrophobic or out of breath. The head end doesn't completely seal anyway, it's just held in postion by a square of Velcro.
In the morning, I discovered that I had a few issues with condensation down on the leg end of my sleeping bag. It certainly wasn't enough to disrupt my sleep (I slept well!), but I did note that the top half of my sleeping bag was damp. I have on occasion had this issue with my British army bivvi, but not to this extent.
At this point, I think it's important to balance this criticism by saying that most bivvi bags will be susceptible to condensation. There are many variables to account for such as weather, location and physiological elements of the user (i.e; are you a warm sleeper/full of hot air!), but you will always have some kind of issue with condensation. I've even heard people complain about condensation in high end bivvi bags from Gucci brands.
It occurred to me that I'm generally a very warm sleeper and that perhaps it might be useful to drop down from my 3 season bag (my usual bag for winter camping) to a 2 season bag. This was on the basis that; a) I'm a warm sleeper, and; b) the bivvi bag does such a good job at keeping the wind off.
So on my second camp last weekend, I did exactly that and packed an OEX Fathom 2 season bag. The bag was taken well out of it's comfort zone (2-7 degrees) as the temps went down to -2 on Saturday night. I was comfortable in the bivvi, certainly not toasty, but by no means cold either! I was also just wearing my merino base layers and a fresh pair of socks. The crucial point though was that there was much less condensation in the bivvi so I felt that I was on to something here and probably made the right call to drop a season in the sleeping bag.
So there you have it, in conclusion, I'm happy with the bivvi bag. It's not the best that you can get, but neither is it the worst. I am currently thinking that I will keep this bivvi for the colder months and revert back to my british army bivvi or a lightweight option for spring/summer use. When I do use it though, I will be confident in the knowledge that I can drop down to a lighter sleeping bag.