I too have been contemplating a gas tankless to replace a gas tank.  Mainly as others have said increase space.  I have a 1000 ft basement and mine sits towards the left side of the area but in the middle next to the furnace.  It would be great to not have a tank just to make the furnace area smaller so that I could one day put some sort of removeable surrounding around it.  Did not really know about the de-scaling as I have not thoroughly researched.  I am on city water and do not have any sort of water softener or reverse osmosis type system.  So if one is to install a water softener as well as a reverse osmosis system I assume that it would reduce the scale.  I know that routine maintenance for descaling should always be done at regular intervals.  And I plan on doing just that.  But I would think that it would really make your tankless system as well as other appliances last for many years to come.  Especially over a tank water heater.  Correct?