In reply to tew:
We've got one, very hard water gets transformed into very soft water, uses a bag of salt (£17) every 6-8 weeks.
If I could travel back in time I would definitely still buy the water softener, but wouldn't bother connecting the toilets to the softened supply. Our kitchen tap also runs unsoftened water on the advice of our plumber, he said lots of people don't be like the taste of softened water, this means the kettle does still scale up.
The cheapest softeners regenerate based on a timer, more expensive ones measure how much water has flowed through them so only regenerate when needed.