Interesting. I've used my old Thermolite (Thermolite1?) with a fleece sleeping bag liner with good results in warm weather, but I haven't taken it into much wind. I was wearing socks, long johns (top & bottom), and a fleece cap. I guess if it were really windy, one could duct tape the vent closed.

Perhaps the advantages of the Thermolite are:
-it can fit back into it's original container
-more durable
-vented (both an asset and a liability)

The Heatsheets bivvy is:
-smaller
-lighter
-(and apparently) warmer

Does that square with the experience of others?