I tried a sleeping in a hennessy hammock and just about froze. Your weight compresses the down in the sleeping bag and it loses it's insulating properties. Apparently, the insulation needs to go below the hammock. But, I love hammocks for lounging in warm weather.

I've not tried a bivy either. To me it seems like just a moisture resistant cover with a bug shield for your sleeping bag. Which doesn't seem that bad. I like the idea of it being a lightweight alternative to a tent.