Not sure if they actually save any money. The argument is that you are not burning gas to maintain the heat of water when you are not using it. If you are heating your house, any heat lost by the water heater goes in to the air in your house, and eventually settles in to the overall energy balance. Your furnace has to fire slightly less. There may be a good argument to the contrary. I'd be interested.
Aside from the up front cost, the other consideration is maintenance costs on the heat exchanger.
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