I tested it on a recent eastern Oklahoma Ouachita Trail overnight trip and it worked well ... under optimal conditions. The temperature ranged from about 60 degrees at 9:00 PM when I set it up to about 40 degrees in the morning. It was clear and dry.
I took a few pictures but don't know how to post them here. I guess I need some sort of hosting service?
1. I need to close the bottom, either with some form of attachment or permanently. As I feared, it rode up on me overnight and the bottom of my bag was out on the ground.
2. There was absolutely no condensation. Of course, in this weather it wasn't much of a test.
3. Polycro was a bad call for the 2x2-ft section in the head area. It ripped almost as soon as I opened it up and tried to drag it just a few inches with my pack sitting on the polycro section. I'll replace that section with more of the soft Tyvek.
4. There were no issues with twisting the bivy up while tossing and turning. My pad is pretty narrow so any turning over has to be pretty deliberate. The inside of the Tyvek is pretty grippy and the pad doesn't slide around at all so the whole things stays oriented correctly.
5. I like having the 4x5-ft floppy head section. It's very versatile. I lay it out at 5PM at a campsite beside a river while I lounged and snacked and played with different ways to keep the fabric over my head but off my face. In one configuration, I put the end of the flap on top of a 2-ft high rock pile people used as a seat and held it in place with another rock. When I set it up again later that night I thought about building a tripod out of sticks but it was crystal clear so I didn't bother -- I just left it down while I watched the stars and sometime in the night pulled it over my head when it got cold.
I'm confident I would have been fine in a light rain. If it had gotten bad I would have had an uncomfortable slog about two miles from a shelter, but at least all of the river crossings were done before I stopped. Testing under heavy conditions will have to wait for stormy weather and free time to overlap.