Nest claims to save people, on average, 19.5% on their heating and cooling bill. If you're paying $50 per month for heating/cooling and you buy a Nest, you're now paying $40 per month instead.

Over a few years time, Nest quickly pays for itself, saves you money every month, and reduces your energy footprint.

It doesn't seem like a "massively over engineered pretty device that very few people need" to me.