Hennesy Bubble Pad
I just got one and have used it down to 45F or so in testing in my yard. I taped a thin piece of fleece on the up side so as to put some airspace between myself and the radiant part (you need air space for the heat to radiate in) and to keep the moisture problem at bay. It works well since fleece doesn't really compress well and I was using a cheaper 30F (maybe) bag I own. Actually I was a little too warm so I think it could go to 40F or maybe 35F pretty easily. I'm thinking of sewing a lightweight fleece 'sock' for the pad for these cool but not cold nights. I have not backpacked with this setup and was also wearing just a cotton shirt and sweat pants. In true camping setup I'd be wearing more appropriate clothing for the temps.
During the shoulder seasons, I use a windshield sun visor as a pad. Its really warm, one side is aluminum like a space blanket.