I've never had a heated seat on a bike before but I do suffer with lower back problems which isn't helped by getting cold.
Aftermarket heated car seat covers help no end in the car in winter, same with the heated car seat cover in my jacket, so, although the bum doesn't get cold like your extremeties on a bike, there is a lot of flesh there & a lot of blood flows through the area.
I figured for the same price as a set of heated grips it was worth a go.
Todays ride wasn't particularly cold at 5-10*C (little wind, murky & damp) but leaving the heated seat on low did have a feeling of loveliness about it.