In my opinion, anything more than a "basic bivy" is not worth it since many 1-person tents now weigh the same. By the time you get a fancy hoop bivy you may as well do a tent. A good bivy is NOT cheap. You could find a "basic" bivy on sale for $150. I have done many 10+ day trips with only a bivy, in the Sierra. If you anticipate lots of rain, I would not use a bivy alone. Under the right conditions, a bivy is great! I LOVE being able to just plop down anywhere, see the stars and still have good protection in case of a change of weather. It takes a while to get accustomed to sleeping in a bivy. And if you are pinned down in weather, a bivy is like being confined to a coffin. A good, basic, mountaineering bivy will protect you from the weather. At 1 pound, 5 ounces, it is less than an equivalent one-person tent. Tarps are fine if always backpacking in trees. I do a lot at higher altitudes. In fact, in a high wind, a bivy is more secure than a tent. I do think, that you need to be an experienced backpacker to be mentally comfortable in a bivy. Most beginners would be freaked out.