Instantaneous water heaters originate in Europe and Japan because their energy costs are very high and their cleaning habits are different than those in the US. An electric tankless model offers no combustibles, no standby loss and take up little space.
In an area like the Tennessee valley Authority where electric rates are very low there can be some good operating costs. Elsewhere however, electricity is 3 times more expensive than combustion fuels, so gas and oil are still a great bargain in comparison to electric heating appliances. I offer a fuel comparison computer program on my web site to do the comparisons.
One downside of the instantaneous heater is the construction. The small tubes can rapidly collect scale in hard water areas. European and Japanese owners are used to having the manufacturer service crew take the unit apart each year for refurbishment. This is unusual for US owners; they may just let it go till it fails.