I had the Entourage PocketEdge dual screen reader for a short time but returned it for a variety of reasons. It was quite heavy (about 24 oz) and the battery life was limited. Even if you shut off the LCD screen and used only the e-ink side, the battery would only last about 11 hours, and much less if you used the LCD screen either by itself or in combination with the e-ink screen. Amazon may find a way to lengthen the battery life, but still you'll only have a few hours compared to a few weeks with an e-ink reader.
Still, it's an interesting concept if it's done correctly.