I Like It
I have a simple heat-only natural gas furnace. The dummies that ran the wire for the thermostat only ran 2 conductor, so I don't have the option of running the fan only which would be nice in the summer. (I don't know if the Nest even lets a person do this).
Anyway, I bought it because it looks cool and my greenie relatives go "Ohhhh, cool, a Nest!". It's in a new house so I don't know if it is saving money or not. My setup is low-tech, my house is new and energy-star rated, so the impact is likely less than using it in a less-efficiant house with more complex HVAC systems.
There are lots of new systems out that have colour touch screen setups that are from traditional manufacturers that will work with more complex HVAC setups. I think the Nest is for someone with a traditional, single thermostat - single zone, system. Maybe they will get more advanced as time goes on - I can tell you that if they do, it will be a gigantic headache for them. There are just too many weird setups out there to expect people to take these home and re-wire their own HVAC.