The main use of them is for electric water tanks. I went home last night and did some math, my gas is worth $6/GJ and is going up $1 next year due to Canada's Carbon tax plan. My water heater uses about 12-15 GJ of heat/year ($60-90). A 40% reduction could save $24-36/year in my optimistic scenario.
The equivalent amount of electricity is worth about $45/GJ - $540-675. Obviously, electric is a lot more expensive to operate then gas. But a 40% reduction is worth $216-270, a much shorter payback. Electric rates vary, but clearly its a viable way to reduce electric bills, assuming you already heat water wiith electric.
There was one other scenario where it works; the case of someone disconnecting gas and going straight electric. You'll pay more for fuel/electricity/heat but avoid the connection charges - $25/month for me ($300/year). This is only viable for people who have heat pumps already and the gas line is only for hot water. In that case lets say I spend $675 for hot water, subtract the 40% so its $405, subtract the basic connection and I'm down to $100. The numbers are getting pretty close to zero/break even. My insurance will be reduced if I cut off gas, another savings. Electric water heaters are cheaper to buy, that adds up too. We all have differing charges for connections, my numbers won't be the same as yours. I'm not convinced this plan is for everyone, just an interesting thought.
For interest, a GJ of gas emits 50 kg/CO2. A GJ is slightly bigger then a mmBTU, 10 therms to the GJ (its actually 10 therms to 1.05 GJ). Over 20 years this device might save 5 tonnes of CO2 for me. I am unlikely to see government incentives as the cost/tonne saved isn't that great for me, but maybe the policy makers will disagree.