This is the first year the I have used a hammock extensively, they can be set up on the ground as you would a tent if there are no trees available.
My Hennessey has given me some of the best nights rest that I have had in the woods, includng during a rain storm that dumped over 3 inches of rain in 1 night and flooded out all of my tent based friends. I was the only one not drying my gear most of the next day.
Also, the whole not sleeping on the hard ground seems to be easier on my ever aging body in the morning
. The nights that I spend in the hammock were around 40 degrees, and I found that putting my sleeping pad under my bag did help keep the warmth in.
The only downside of the hammock that I have found is the lack of storage space for your stuff, I ususally set up a tarp near my hammock to keep my gear dry.
PS Thanks Doug for stepping in and stopping the building flame war. There is good information in this thread and I would hate to see it degenerate.