Not hot at all. And I tend to sleep hot naturally (i.e., always trying to cool down). Mind you, we're rocking pretty sweet central AC (which of course is now off), so it will be interesting to see how it will feel next summer.
I realized the other day that I no longer have to (eventually) roll onto my left side because of an achy right hip. This happened a few weeks after getting the Endy but I didn’t make the connection, I just... never had the discomfort again and stopped thinking about it.