These things have the potential of saving energy, but even with today's energy costs, it will take a long time to break even, if you ever do. They require regular maintenance, and if you don't perform it, they will decrease their efficiency and the amount of hot water they can produce.
If you size it for significant flow, you'll be buying a very large unit, and may require a bigger gas main, piping, and significant flue, or a major increase in electrical service. Just 2.5 gallons per minute needs nearly 100K btu or more, depending on the input water temperature. Try filling a tub or a washing machine. If you have one thatcan actually handle a large flow (they all come with a flow restrictor which can make your shower less comfortable), when you run a little bit in the lavatory, if you don't have a tempering valve to moderate the temperature, you may end up with water extreemely hot and then scald yourself. At least one brand modulates the heat output to try to match the flow, but it still can create problems. That adds to the complexity, and therefore the cost to keep running or repair.
Comments have been made about the water heater having to run several times during the day to keep that tank hot. Well, thatmay be true, but not necessarily. Depending on the tank and how well it is insulated, that may not be the case. If you happen to have a boiler, using an indirectly fired hot water tank relies on the boiler for thatheat...some of them are in the mid-90% efficient range and the better tanks are rated at a loss of less than 0.25 degrees per hour. That's 6 degrees drop over an entire 24-hour day...it may not even turn back on.
A tank type of heater matches the more common US usage patterns, is cheaper to buy, install, and maintain. Spend a little bit more for one that is well insulated. You'll come out ahead, and be more comfortable in the shower, and your washing machine and tub will fill faster, too.