I just moved into a house which has an on-demand water heater. It is powered with natural gas just like our furnace is.

I don't have a brand name for the one we've got, but it works fabulously. It doesn't save us any time getting the hot water from the garage to the shower, but when the hot water does reach the shower, its temperature is very consistent, even when other taps in the house are being used. I can safely shower while a laundry load is being run, for example.

Its noise level is noticeable, but since it's mounted in the garage next to the furnace, it's not a factor for us. In terms of volume, I'd say the on-demand system is louder than a conventional gas hot water tank would be while heating the water, but it's significantly quieter than a furnace.