Just to make it clear for everyone, it's not e-ink display. It's a transflective LCD display, which isn't the same thing and it's not nearly as efficient as e-ink either (although I have no idea if e-ink would even work properly on a watch). My guess is it's around 2x more energy efficient than a color LCD watch. This is what they're using for Pebble:
http://www.sharpmemorylcd.com/1-26-inch-memory-lcd.html