Bivys work great in good weather. They suck in any sort of precipitation unless you pitch a tarp over them. Maybe a bivy plus tarp is lighter than a tent, maybe not. Without the tarp over your bivy, you can't change in or out of wet clothes. I have a nice one but use it pretty sparingly. Given the choice between a tarp plus a sleeping bag and a bivy plus a sleeping bag I'll bring the tarp every time.
