I have a Hunka but have never really got on with it. Getting in and out of it is a chore, in the end, I found that to get into it comfortably I had to stand in it first, pull it up to my head and then lay down, vice versa for getting out of it. If your head end is under a tarp you might have to wriggle around a bit to get into the right position. What it lacks is a zip down one side.
So what I did was to buy a Snugpack Stratosphere, much better, a raised space for your head (my boots, one each side of my ears) and a long zip down one side, a much better design in my opinion. One matter has to be thought of, do you use it by itself, (if it rains before or as you are getting in you have an immediate problem) Do you also take a tarp with you (I used a 3.5 Alpkit one) or to try to get organised in such weather under an umbrella? By the time you have packed the bivi, tarp or the umbrella the weight is the same or more than taking a tent and takes up a similar amount of space.
I use my bivi when I go away to a maximum of 5 days and want to wild camp, when I'm confident as I can be that the weather is going to be fine, when I want to stealth camp (that is for me, when I want to bed down near to the pub I've just frequented) and when I just want to walk, sleep, up and go!
You also have to think of another problem as well, once in your bivi what do you do with your rucksack? When I use my tarp I just push it under a corner to ensure that if it rains it will stay dry. If I use my brolly I put my waterproof rucksack cover over it and I have a piece of fishing line attached to a cheap audible alarm which is attached to the bivi. I don't want my rucksack to go walkies in the night without me knowing about it, theft is a possibility if on a site.
I do like my bivi but I also know that I have to live with its obvious limitations.
I from this Sunday I am due to go on a 12-14 day hike from Keswick to Edinburgh so I will be taking my tent, not my bivi. I hope some of this information is helpful.