I have the Ultralight Backpacker model and have been very happy with it. I have been down to 40F weather with it, but nothing below that. I was in a 32F down bag with no sleeping pad and was very comfortable. I remember seeing a post a while back where someone was talking about attaching the more substantial AMK emergency bivvy to the bottom to reflect heat back up, so I might try that on a trip this winter.
I like the idea of the Clark hammocks with their sleeve for a sleeping pad, since sleeping pads like to slip around on you in a hammock, just like a tent. Smear some silicone caulking in a few spots on the bottom of your pad, and you can avoid that problem.