We put one in when we built the house in '92. The city water tests hard but tastes awful. The whole house system has needed the pellet canister replaced twice (they compact down over time). We upgraded the head unit from a mechanical unit to a digital one (computer) about 10 years ago. Much easier to set and change as needed.
Buy the salt pellets from Home Despot as needed. I usually add a 60# bag every 2 or 3 months. Run the backwash for about 15 minutes every month.
Longevity of the electric elements in the water heater are improved as are the dishwasher, clothes washer, etc.
With the HE washer and dishwasher, we use less soap per wash as well.