In the case of Nest, I suppose I can imagine that I took off on a trip and forgot to turn the thermostat down. But I balance that against the cost, the "it ain't broke" thing, concerns about decreasing the reliability of my heating system in a cold area of the country, and a sufficiently variable schedule that I challenge any non-omniscient device to learn my patterns.

There are some useful things like leak detectors, low temp warnings, and general piece of mind visibility into what's happening with my house when traveling that I've installed. But most of the non or minimally automated things (I do have a couple wireless X10 lights) around my house work well enough that they're pretty far down on the list of stuff that I wish I could improve to make my life easier/better.