E-Ink displays are much more energy efficient than LCD. Once a page is displayed on an E-Ink eReader, no further energy is required to maintain the display, unlike LCD displays that require continuous power supply. In fact, if the batteries are removed from an E Ink ereader the last page of text or image displayed will remain intact.
Due to the low power consumption of portable devices, the higher energy efficiency of an E Ink eReader may not reduce your carbon footprint by a noteworthy amount or even, for all the eReader in the world, collectively save the planet from global warming. However, the miserly usage of batteries by E Ink ereaders compared with LCD readers does produce one very important result: battery life of E-Ink ereaders is measured in weeks rather than hours for LCD based ereaders. Some of this advantage may be and often is traded off for smaller battery size and weight.
The combination of long battery life and small battery size help to make the E Ink reader an ideal travelling companion for long trips, particularly ones where using your charging cable or adapter may be difficult.
The flip side of persistent display is that current E-Ink displays are very poor at displaying video.