They are incredibly useful. I found them essential for the primary school run - or rather 15 minute walk, more if one of the boys had a meltdown because I couldn't control the direction of the horizontal rain to their satisfaction.
The problem with raincoats is, if you wear them in actual rain, the water runs onto your legs.
Waterproof trousers are useful when it's cold and windy or snowing, as well, as they keep your legs quite warm.