i have a solo tent but i like the idea of a bivy. never used one tho. super light and packs down to nothing. i hear condensation is a huge problem in most of them so you will wake up with a wet sleeping bag. my biggest issue with a bivy is the price. you get less and costs as much or more than a nice back packing tent.
I have heard the stories about condensation as well, however I found some MSS Gortex bivys that the US military uses, and they run pretty cheap (like 60$) on surplus websites. Im prolly gonna pick one up later this week.
The reviews I have read said that the Gortex ones dont have the condensation issue that the silnylon ones have.
I had thought about a hammock, but my only concern with that would be what if you dont have suitable trees to hook up too? The tarp I can set up on anything with trekking poles, and the bivy i can plop down anywhere.
Lifetime backpacker here. I've used both and the bivy is nice when the conditions are absolutely perfect. I live in Texas and the mosquitoes are terrible at certain times during the year. I'm much more comfortable in a tent most of the time.