DoE says they breakeven is 2.5 - 7 years. That's in the US, imagine how much of a slam dunk it is in countries with carbon prices and higher energy prices.
These devices actually require a similar amount of water to be moving through the drain pipe/heat exchanger and the cold water line that's refilling the water heater. So they're really only effective during showers.
Perhaps only if you fill a large tub or a pool every day.