I actually use the thermarest all the time. Even in summer the hammock can become quite cold since the heat transfer through the bottom and sides is so great.

The other benefit of taking the thermarest along is that if you need to use the hammock as a bivy then you have a bit more cushion between you and the ground.

The hammock works better in warmer weather that's for sure. I have some of the additional pieces that extend the temperature range significantly and they work well. The biggest problem is the sliding off the therma rest when it's cold out. In the warmer months it doesn't really matter much.

~Jamie N