An instant hot water dispenser is a kitchen appliance that dispenses water at near boiling temperatures. There are hot-only and hot and cool water models, and the water may be filtered as well as heated. Instant hot water dispensers for kitchen use became popular in the 1970s. Come to think about it, many times throughout the day there is a need for instant hot water, see list below. Boiling water by any other means takes time and often you boil more water than what is needed which is a waste of energy.

HOW DO THEY WORK?
When the handle is pressed, cool tap water flows into the tank and displaces the near-boiling water, which flows out of the spout. On releasing the handle the valve closes and hot water stops flowing. The cool water is then heated to up to about 94 °C (201 °F) ready for the next use. Some tanks feature a 750 W heater that delivers up to 60 cups of water per hour at a temperature set by a dial. Some models are equipped with an advanced filtration system that can reduce chlorine taste, odor, sediment, toxic chemicals, heavy metals such as lead, and dangerous minerals.
The dispenser unit is connected directly to the water supply. The faucet is mounted to the countertop near the sink. The thermally insulated tank is, in most cases, mounted underneath the counter. Some models have a surface mounted tank above the sink. Better models have a built-in water filter and a thermostat to adjust the water temperature. Instant hot water dispensers should be installed by a qualified plumber.

THEY COULD WASTE A LOT OF ENERGY
Water is kept constantly hot at all times. When there is no call for hot water, energy is wasted. The amount of waste depends on how often the unit is used. The waste is the greatest if the unit is used only occasionally, it is more of a convenience than anything else. It maybe a different story for a family with small children as hot water is a constant need.

OVERALL ECONOMICS
* Instant hot water dispensers are expensive to purchase. The last time I checked the local home improvement centre they had only one model on display which had a price tag of $219.00. The unit was able to deliver 40 cups/hour of 88°C (190°F) water from a 1.26 L (1/3 gallon) stainless steel tank.
* In all likelihood you need a plumber to install the dispenser which is also expensive.
* It is fair to assume that the unit is "idling" most of the day (and most likely during the night as well) wasting energy until it is time to prepare meals.

SUMMARY
Unless there is a constant need for an instant hot water dispenser or if economics are no issue, go for it.
If not, the good old kettle will still do the the trick or, in most cases, hot water straight from the kitchen tap!!