Since I bought my pebble and got used to checking that for the time and text messages I have increased my phone's battery life at least 50%.
The major drain on a phone's battery life is screen (backlight) and OS, and when we pull our phones out with the backlight on full we drain a _ton_ of battery life.
I love the Pebble because it's a developer-centric reinvention of the idea of a "watch". Everyone is invited to re-imagine what a watch is. And it has a 7 day battery life.