>>1740751>Is it a replacement for a tent?This one. As the anon you replied to said there are two types of bivy. Heavier ones made of gortex or something similar that are weatherproof on their own. IMO these things suck unless you are mountaineering. OP might be ok in one if he's planning single night trips or whatever, but even then a tarp will be lighter and pack down to a similar size, if not smaller.
The other type of bivy isn't meant to be used alone, but to add a little extra protection when using a small tarp. They stop splashes, dust, and provide bug protection. I often use one of these, especially on trip when I don't think I'll need to pitch my tarp often as they are great when cowboy camping.