Patrick, I was a bit surprised I did not run across one of us out their. My guess is a few days before with nice weather I would have. I only saw a few hardy shelter campers in the 40 miles though in the rain and snow.
Yes I did use my own quilt, but I modified my original 3 season quilt. I cut the length down 6" (cut out one 6" baffle section) and cut 2" inches from the width. This took the weight from about 20.5 oz to 18.35 oz. The length is just long enough for me. I removed the side width without loosing down so it lofts just a tad more.
The condensation story is interesting. I intentionally used M50 for the bivy as I believe that internal condensation management is more important than splash management. The first night was a real challenge. The fog was thick and it rained all night. My pants legs bottoms were very wet and most of my other clothes were moist.
I slept with the bivy fully zipped, mostly on my stomach with my head turn to the side. During the night I noticed moisture forming by my face. When this happens in the past with my MLD superlight I would open up the hood to help the moisture. I intentionally stayed buttoned up with this bivy to see what would happen. Well I woke up with significant mositure inside. So much that my quilt was damp. I was a little disappointed, but this was an extreme situation.
The next night was almost an exact duplicate with a few exceptions...my bag was now starting out wet, my legs were dry because I hiked in shorts only, and I slept with my head out of the hood. It again rained all night and the ground around me was completely soaked, compared to damp for the night before.
What happens was even more surprising. The temp was mid 30's and I started out chilled as my bag was maybe 50% of its orighinal loft. When the morning came I was warm. My bivy was bone dry, I mean BONE DRY and my bag had largely dried out too. I would say this was probably the best night for moisture management in a bivy I have experienced.
I wish I knew the secret to what made the difference. The second night showed me that M50 can indeed make a great bivy material for challenging rain/moisture. Does sleeping with your head outside the hood really make that much difference?
One other item of note for moisture management is the rain skirt. Where else can you cut weight in such a significant way with little or no performance loss for small cost. During high wind and rain my pants bottoms got wet, but the rest of me stayed dry. The legs dried easily in camp. Consider that most rain pants weigh 5-10 oz and that a kilt is just over an once and can be used as a small tarp to sit on it is definately a keeper.
My rain skirt is real similar to the cloud kilt. I would highly recommend folks considering using one over rain pants.