I have to disagree with the post by hydronicnetwork.net.
I have a tankless hot water heater in my lake home in Minnesota that we use year round. Initial costs are higher than convential heaters but they are economical to operate and could provide hot water until you either ran out of gas or water.
I have disassembled my tankless hot water heater twice in the 13 years it has been in use and found no build up in the tubing (which is not narrow).
We may be lucky to have water that will not contribute to build up, I don't know. What I do know that I will be installing a tankless gas hot water heater in my other lake home in Arkansas when we move there for the winters, when I retire.
You wanted input from someone who actually has and uses a tankless hot water heater. This is my input and my opinion.