Thanks sharpmaker for sharing your experience. I definitely understand your points, and from an efficiency standpoint, a heat diffuser is probably not the best way to cook. I use mine with my cast iron dutch oven and heating it up takes a long time, and the instructions for the copper heat diffuser is to not heat it up on high, so it's slow and steady the whole way. And I would never dream of using it to boil water.
With that said, I often use it as a defroster plate and always use it now with my dutch oven. I would never use it without now. With my gas range I used to have problems with evenness of heating, but no longer is that the case with my heat diffuser. So while it may not be a winner in terms of efficiency, IMO it is very helpful when cooking anything where you want more evenness of heating.