To give you my experiences:
I've just been using the Dutch hooped bivvy for a week doing the Frontier Intermediate course.
The bivvy certainly adds some warmth to your overall set up, but it isn't the difference between cold-just right-sweltering. That's going to be down to the rest of your sleep system: mat, sleeping bag, liner and clothing. And while you may be very warm when you get inside, as I frequently was, the ambient temperature drops dramatically around 3-4am.
The temp last week was variable, I was mostly camped inside our shelter and occasionally next to a fire. The bivvy has good adjustability ranging from being completely cocooned, to having the zips partially open, to having the zips completely open and laying the hoop back flat like a baby carrier.
I would absolutely follow Pattree's advice and keep your limbs covered. I made the mistake of sleeping in my undies with and without a tee. While I could manage my temperature fine, I have, I would guestimate, 30-40 mossie bites as a result of sleeping with the bivvy and my bag open, exposing my delicious limbs.
We had a biblical storm on Thursday night and overall I was very impressed with the bivvy's water repellancy. BUT - if water is allowed to pool on it, and if there is something inside making contact with the shell, like your sleeping bag, it will get wet.
As it was, only the foot of my sleeping bag got wet which was no real problem.
Lesson learnt: air and then drybag your sleeping bag til bed time.
Best option is to lay the bivvy on its side (zips down) if it rains.
Things to note:
While the bag is quite roomy, that room runs out quickly once you shove a sleeping bag and an air mat in there. It's designed to work with a closed cell foam mat on the outside.
I was in there with a very thick air mat, an air pillow and a down sleeping bag which really didn't leave a lot of room for wiggling. The toe box was quite tight. Lying on my back I had about 1-2 inches of space from nose to roof when enclosed. But a lot more width-ways.
Once you seal yourself in, it is basically a coffin. In some respects, that's a good thing; you feel very protected from the elements. But it may trigger or reveal some claustrophobia. I'd definitely get inside with all your gear at home before hitting the field just so you know what you're dealing with. Undoing the zips a little will provide a reassuring draft.
There is a decent amount of headroom at the top of the bag, but don't expect to get a 45l pack in there. There was ample room for me to keep a pillow, change of underwear, a fleece, torch and a small drybag with electricals.
Pegging it out isn't essential but it does help provide some structure.
As you may know/have read, the zips are pretty crap. Go slowly with them and be mindful of them being under tension or getting fabric caught in them. And keep them covered with their flaps.
Hope this is helpful,
F.
