This allows me to measure the temperature inside, outside, and get the relative humidity (not nearly as accurate as the $20 honeywell sensor that they're using, but, it's close enough for my needs). I then built a simple website using mrtg (for temperature trending) and a ruby script that checks the temperatures versus what the set points are and mounted the raspberry pi's in various locations around my places.
My "Controller" nodes are a beagleboard with a 4 or 8 channel relay board attached that allow me to turn on or off the individual controls on the furnace. It works well with my two stage heat pump and fan at my home, but, I need some work to get it 100% at the hackerspace and at the farm.
I mainly did this because I needed something that allowed me to cover more rooms than the Nest (and I'm adding duct dampers and fans to my heating system, so I can selectively heat and cool more rooms to better temperatures).
That, or a put together a nice board that you can plug into the RPi to interface better with it maybe :)