I tend to fit them on every boat I work on.
Nothing is perfect and certainly not for £150, but it does give you an idea about the state of charge of your batteries, in a simple to understand "fuel gauge" type presentation that most family members can understand.
To be honest, I normally fit an ammeter as well, as I think its useful to see the amount of current flowing into and out of, your batteries.
Very useful for cycling batteries between 50 and 80 pct charge to avoid over discharge and running engine/generator longer than necessary, but this will depend to some extent on your particular installation and charging habits.