Ditto. We've had one for 6 years and the savings in propane is pretty significant versus the tank type water heater.We've had propane heated, on-demand hot water for 11 years. I have no intentions of going back to a tank. We've never run out of hot water. It has no problem heating our icy well water. It's very efficient.
To accomplish immediate hot water at the sink you wouldn't use a tankless water heater along with a traditional water heater, you would use a temperature activated valve like the Hot Water Lobster.I believe this is nothing more than an 'on demand' water heater which heats the water as it passes through. IMO, this is not very efficient if the main, traditional style water tank is in place as the main tank continues to consume resources. The better application is to replace the hot wate tank with either a tankless heater for the entire house or several small units scattered about.