On the "debit" side.
= dietary sodium intake increases, especially if you prepare home cooked meals most of the time and make your own coffee, tea, seltzer, canned food, etc. Note: This is not the best forum for a debate on how much sodium is "good" or "bad". However, it is certainly true that sodium content of softened water is higher than in unsoftened water.
= rinsing off after a shower or bath is made more difficult and the softened water often feels "slimy": this increases water consumption.
= the increased salinity increases osmotic stress on plants watered with softened water.
= environmental burden through the softening product life cycle includes energy needed for: equipment and plumbing apertenance manufacture; distribution and delivery of softening equipment; electrical current needed to run valve actuators, timers, warning lights; fuel needed to make purchases of salt and other consumables, energy needed to produce the salt and resin consumables; and, if you live in a community which is reliant on reverse osmosis to produce potable water from recycled wastewater, the energy needed to run that equipment is increased by the salt from your softening equipment.
= domestic wastewater has increased sodium chloride content, potentially making it incompatible with certain types of onsite wastewater treatment systems and increasing slightly the salinity of freshwater stream and lakes. This has greatest significance in regions where evaporation is high and precipitation low, leading to low flow or intermittent streams.
On the credit side.
=energy efficiency of hot water heaters is favorably maintained by reducing the formation of scale on heat exchange tubes or elements.
=reduced soap and detergent consumption
=reduced scum formation on bath and shower surfaces (hence reduced need for cleaning agents and water use for clean up tasks).
=a carefully set softener can result in a better "feel", with no objectionable sliminess, and, to some, provides an improved tap water taste.
Thoughts of compromise:
It should be possible to soften only the cold water feed to a domestic water heater that supplies hot water for dish washing, bath, shower, and clothes washing functions. By adjusting both the output temperature of the hot water heater, as well as the extent of softening, it should be possible to gain the benefit of scale reduction where energy efficiency is at stake, with minimum salt consumption. Dilution of even a highly softened hot water flow with unsoftened cold water would avoid the "slipperies". Point of consumption filtration can be used to remove undesireable odors from an unsoftened cold line for cooking and drinking. Just this simple approach should cut salt use roughly in half and eliminate several of the "debit" issues.