Are you going to miss having the ability to sit up in your shelter? Do you not need to store gear inside? Do you mind spending days trapped inside your bag when it's raining cats and dogs? Are you able to get in and out of it in a flash and then dry off before getting back into it if you've gone out to water a bush? Are you a mountaineer who stays on high narrow ledges? A bivy might work for you.

There are two kinds of bivy - the heavy kind for mountaineers that can't use a tarp for cover, where they are expecting to survive a rainstorm in just the bivy. And the lighter kind that provides some protection from bugs and spatter that makes it under the edges of a small tarp. If you go with a mountaineering type bivy and you are not mountaineering, you're probably going to be sacrificing the space and comfort of a tent for no reason, since some of those bivies weigh as much as a double wall solo tent, and in many cases cost more.