They have over 100 people, and if rumours are true have quite significant backing. Which is particularly interesting as it's hard to see on the surface where their margins come from on a device most could consider as novel and expensive.
Though there are potentially gains to be had from a smarter thermostat, the headline figures in their white papers will most likely never be realised in the real world. Most reviews and bloggers seem to get caught up in focusing on the simple heating and cooling experience.
The company on makes sense once you look at it in the context of the larger market. The silent but important features are zigbee integration (current unused), and excessive processing power. We are just on the cusp of huge smart-grid rollouts in many western countries. British gas in the UK has decided on the zigbee standard and are starting to roll out over 1000 new meters a day; with government backing for an £11bn rollout to 27 million households by 2019.
In light of these rollouts, the energy companies will be looking to capitalize on their investment (which will be mostly funded by the consumer, via higher bills). The hardest part is figuring out what the consumer face of the smart grid should look like. Expensive 'home hubs' and touch screens are a red herring - the future is distributed (every household members phone etc), yet you still need a link between the rather 'dumb but integrated' meters, and devices in the house.
In my opinion nest's game plan is to become that link. Your thermostat controls around 50% of the energy usage in your house. Eventually it has the potential to control 100%. With smart GPS integration into your phones it becomes realistic to have houses that react silently to it's various inhabitants patterns and blend those needs with the energy grids demand levels; now this is a valuable proposition. If you're an energy company absorbing several billion because of government pressure, suddenly the hardware cost of a nest doesn't seem so bad, especially if it can be offset or laid off over time.
edit; footnote - All figures are rough (off the top of my head)