I have a chest-zip ptarmigan. As others have mentioned, the chest zip is a pretty big inconvenience, especially in nasty weather, but since you only have to do it twice a day, it's not that big of a deal. After a few trips, you get used to it, just like you get used all the other compromises you make to reach SUL weights.
I was dealing with heavy thunderstorms this last weekend and trying to get into the bivy while staying clean and dry does require a bit of gymnastics, and isn't 100% possible. When it's dry I just sit in the dirt and brush my butt off sliding all the way in. However, when it's muddy, trying to get your feet in while sitting on the bivy head and keeping your entire body under the coverage of a small 5x8 tarp (half-pyramid pitch for amazing views) during intense rain requires some crazy gymnastics.
It also probably didn't help that I was pretty hammered. I was up at a remote (9mi in) natural hot springs, which is basically one big party most summer and fall weekends. A clothing-optional, alcohol-required trip. Perfect for when you can't find anyone to get out with for a few days, but still want to be social.
My 24-48hr 13L 10-lb SUL backpack (w/food+water) was a huge hit, even with the ranger ("are you up here for the day?"). It was also a great icebreaker with the ladies where numbers and um.. more.. ensued... win! I flew up the 9mi 2700' trail in 3 hours and passed many poor souls who had pack weights definitely in the 30-40lb range. I'm sure I converted a few that weekend.