Sony SmartBand Talk and YotaPhone 2
E Ink is a popular display type for e-readers due to its low power, easiness on the eyes, and high visibility in direct light. These displays are finding their way into wearables and even smartphones now, too. The YotaPhone 2 is actually the second generation YotaPhone that sports dual displays. The original YotaPhone has an LCD primary display and the YotaPhone 2's primary display is OLED. Both have large secondary E Ink displays on the back of the phone. The E Ink display draws virtually no current compared to 150mA to 400mA (brightness dependent) for recent OLED and LCD displays. Another device that uses E Ink is the recently released Sony SmartBand Talk. The low-power / always-on capability of E Ink does solve a few of the nagging issues of wearable devices. Full color, high-resolution displays are not necessarily the highest requirement for ~1" displays worn on the wrist either. Look for other wearables to follow Sony's lead.
For further analysis on the future of E Ink please see ABI Research's insight: Ecosystem Attrition Driving New Uses for ePaper Technology