As for the Army Gore-Tex bivy cover, I really like mine. Back in October I hiked into the Uintas for 4 days for the elk hunt with some friends and had planned on strictly sleeping in the bivy for my "tent". Our campsite was tucked into the trees so high winds were not a concern and my extra gear would stay dry under a tree. I was told that I would be responsible for everything I'd be needing for the trip and packed accordingly. What I wasn't aware of is that there would be a small tent to share when we got up there. So I slept with my pad and sleeping bag inside the bivy cover while sharing a small tent, this proved to be a smart move as the tent was cheap and some water managed to seep in. My TNF sleeping bag is rated at 15 or 25 degrees but the bivy cover adds another 8-10 degrees to that rating. 2 of the 3 mornings we woke up with a frozen layer of snow on the tent and it got down to 19 degrees, but I was toasty warm even with my lighter bag and stayed very dry. My sleeping pad is just a Thermarest ProLite but I love how compact it is.
What I learned from this trip is that a compact bivy is great for extra protection, but I would not want to use it as a primary sleeping area due to the difficulty in keeping mud and debris outside when entering/exiting the bivy/sleeping bag unless it was for an emergency situation. But there are bivy covers that have more room and incorporate a single tent or trekking pole to minimize that claustrophobic feeling some people get, also allows for easier/cleaner access and room for your gear.
I currently own too many lightweight tents to ever justify buying an expensive bivy setup, but there are many designs on the market to suit the needs of every buyer. Next year for the hunt I will still pack my bivy cover, but will also pack either my Eureka Cirrus 2 or my Cabelas XPG2 tent. If my buddy wants to split the packing weight I will bring the Eureka Timberline or the Walrus Armadillo 4 season.
