Although the Nest was my preference (more on that in a second), it's been sold out since its announcement in October 2011, so I almost purchased the Ecobee Smart Si out of necessity.
But I'm glad I waited because Nest is now in wide distribution from the Apple Store and Lowes.
Nest appealed to me because co-founder Tony Fadell is widely considered to the "father of the iPod" having overseen 18 generations of the ubiquitous device during his Apple tenure.
Fadell started working for Apple February 2001 as a contractor designing the iPod and planning Apple's audio product strategy. He eventually became Senior Vice President of the iPod Division succeeding Jon Rubinstein in 2006.
Nest is dead simple to use. It watches your behavior as you adjust its temperature settings for the first few days, then it takes over from there. Once Nest has learned your heating/cooling rituals it programs itself and takes over from there.
Each time you change the temperature Nest learns your preferences and adapts accordingly. And if it's ever a little too hot or cold and your lying on the sofa or in bed, it's trivial to adjust the temperature setting from your iPhone or iPad using the Nest Mobile app for iOS (free, App Store).