Although it is a pricey appliance, it has to be good for a specific niche.
-Location of the thermostat
(mine is near hall to kitchen-living room...completely bypassed most of time)
-Type of HVAC
(I've a newer Carrier, and noticed savings in bills after setting it to 76F in summer and 65F for winter...without too much spike to ramp up when occupied)
-Need to save energy vs ROI on the nest
(a $90 fully programmable can save you $150 if you don't need ease of use, remote setup-control...)
For me, not worth re-wire, painting area the reduced footprint uncovers, and ... well... it is cool! --