They're much better than the conventional, power sucking water heaters. They only draw power when hot water is demanded. The price of tankless heaters has really gone down in the last few years and make them a much better alternative over tank heaters.
It depends on how much hot water you use at once and how often you use it. They make large models for large households, but they are very expensive. I have a smaller unit, it only uses power when hot water is in use and the hot water supply is never ending. If I go on vacation for a month my water heater costs me nothing to operate while I'm gone, you can get gas or electric units. mine is an electric Rheem brand. it cost about $220.
we love ours (Takagi) because it takes much less room, we never run out of hot water, and we don't hear the hot water tank banging all the time like a conventional water heater. Biggest down side is that you don't have a water reservoir in case of emergency