Well... I'd have to say that there isn't much diffence between a bivy and a bag-wrap topped with netting. It's just put together nicer. My bag has goretex, and the only place it gets condensation is along the inner seems. It takes some fiddling around with the ventilation in the middle of the night to keep rain from getting in, but it's doable.

I used to sleep in a bivy for years, but I've recently switched back to using a tent. The only thing I'd use a bivy for now is an emergency 4-season shelter.