This is a great mattress pad for a waterbed; we got one for our waterbed and have had it on it. We turned the heater completely off; can't even tell. It does an unbelievable job of insulating you from the water; in fact, I almost wish it didn't do quite as good of a job because then I could use the water for extra temperature control and cool myself down a bit in the summer =] However, the electricity costs when this is on the bed are much lower.

Good quality item, works exactly as advertized -- and much better than I expected was possible, as far as the temperature insulation goes.