I have never used a bivvy before. But I remember reading on this forum a while back that there can be issues with water vapor collecting.

I too am buying a hammock. My buddy has an ENO with a screen and tarp tent and its pretty nice. But again my experience is limited with hammocks, unless its lounging in the backyard smile. My plan is to search the hammock forums cause there is so much information on it.
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"To me, hammocking is relaxing, laying, swaying. A steady slow morphine drip without the risk of renal failure." - Dale Gribbel