I've been using the SF bivvi bag for just over a year know and really like it, I'm 5'11, 40- 42" chest and have plenty of room in the bag, probably to much but I hate feeling confined when sleeping and combined with either of the SF sleeping bags have not had a cold night yet. The material is breathable (see snugpak link in first post) although they recommend against sealing the bag completely so as you can still breathe,always a good thing....
very waterproof at 5000mm HH and packs down reasonably small. I've not tried the Alpkit Hunka for a comparison so don't know how they rate against each other...? But the centre zips on the Snugpaks make life a little easier (for me anyway).
I'll be sticking with the SF system for the foreseeable future.