Cost: tankless water heaters cost more to purchase than the tank-type models and are a bit more complicated to install. This extra cost is likely to be offset by the lower cost to heat the water in your home.
Tankless systems are more complicated than tank type water heaters. They are also newer to the US market and there are not as many plumbers who have a lot of installation experience with these systems. The good news here is that I have found the online and phone support from Bosch and others to be very good!
Best time to install: tankless models require somewhat different installation practices than the tank type models. For example, most units require a 3/4" gas line and a condensate drain line. So its best to install such units in new homes. Installing them in existing homes may require some modifications.
You may find that it takes a bit longer for the hot water to reach the faucet. This may result in a limited amount of extra water usage. You may use a bit more water but are likely to save energy.
People may take longer showers because the supply of hot water is endless. But you may notice that it is a little trickier to adjust the shower valves for the even desired temperature.