I picked up a new bivy yesterday. The one I currently have and really like is made by Outdoor Research. It has poles that you can use with it and they keep the fabric off your face. It's cut really wide. It's awesome for backpacking but it was always heavy to take on the bike. Having the bag on my face or being in a tight space doesn't bother me at all. I saw this one at REI a few months ago and liked it. Personally, I have to have a way to keep the creepy crawlys out of my sleeping area. Some of the ultralight sacks from Rab that I have tried are just completely open. Here in Colorado I don't ever have a problem worrying about things snuggling up to me in the night but if I were to sleep in the desert it would be a concern. I wasn't so much worried about having the ability to completely seal the bivy off from the elements with this one. In my head, I know what my strategy is for this year and it's very different than I've used before. I don't plan on sleeping if it's raining. Right now, I don't plan on sleeping much at all but we'll see how I feel near the end of July. One important lesson I have learned in doing these events is to try to make gear do multiple things. If I get in a situation where I need to protect myself from the rain then I'll use my rain jacket as a tarp since the screen on the bivy isn't all that big. Right now I'm planning on just draping it over the opening for short time spans but if I needed to hunker down (if injured) for longer times, then I can always make an elevated tarp with it by using some paracord and trees. This bivy weighs 16oz even with the stuff sack. It's got a huge opening to get in and multiple zipper pulls so I can stick my arms out (or my head) and do things like cook or fiddle with gear while still being protected. I'm 5'11" and this is the regular cut. It's perfect.
I still have my 45 degree North Face down sleeping bag but, right now, I'm not sure that I'll even be packing that. I'll know more when I get to do some testing but I'm hoping that I can stay warm enough in just the bivy to catch a 3-4 hour nap when I start seeing things. With this new bivy alone (even if I bring my sleeping bag) I have cut 20oz from my sleeping system. That may not sound like much but it is. Perhaps more importantly is the bulk that I've lost in the system which will allow me to carry more food in that dreaded 200 mile stretch of nothingness.