I agree with mrs.chappie. My mum was advised to have one fitted as it would save her money being a single person and thus low water user, compared to the water rateable value of the 3 bedroom bungalow she has. It does! and similar to the amount mrs.chappie mentions.
I guess the differentiator is whether you would use more water than the fixed water rateable value. Often single users or couples in relatively large houses will be better off as the water rateable value of the property is high compared to their actual usage.
They aren't money-saving if you have high water usage (so cut down!), not that those in new houses have a choice seeing as they have been compulsory on most new build properties for some years now.