I'm thinking about bivvies. I'd love to try sleeping in one, but as I don't know anyone with one that I can borrow, I'll have to buy my own before I can try it. I briefly toyed with the idea of getting one to sleep in for when I walk the Great Glen Way as it would be a lot smaller and lighter to carry than my tent. But as it'll probably rain and could well be cold and there's sure to be zillions of midges at some point (it is Scotland after all) I think a week in one might be too much. No matter how much I resent carrying my tent during the day, I'll be really glad of it at night. I'll have room to spread out and cook and sit up and read and feel protected from the weather and the midges. I've used it in really strong Icelandic and Shetland storms and it's had no trouble surviving and I've always been warm and snug inside no matter what the weather's doing outside. So the tent it is.
I'd still like to get a bivvy though. A year or so ago, I read Ronald Turnbull's The Book of the Bivvy and found it an informative and interesting read. More recently Alastair Humphreys has had a bit of debate on his website about which are the best bivvies to get.
Once I get one, then I'll have to think about where to take it to get the full bivvying overnight experience.