Here you can find a quite well written review of the Bookeen CyBook 3 eReader by someone who has got their hands on it, and is also familiar with the Sony and iRex products.
I'll let you read the entire review, but here are my highlights:
They identify the difficulty for the user of navigating through a large collection of titles:
"When you have hundreds of files on your card, you will definitely lose your orientation, since the CyBook does not support a folder structure."
Even this E-Ink VizPlex display does not reach the level of quality of the iRex Illiad for this reviewer:
"Now the bad part. Even though the device uses a Vizplex screen, I am quite disappointed in the display quality. Don't get me wrong, it is still amazing. But in comparison to the non-Vizplex display of the iLiad, it does not even look as good. While the display itself might be bit brighter than that of the iLiad, the iLiad is definitely clearer. The CyBook's display has little dark speckles on the white, and the black is not as dark as on the iLiad (white speckles includes). The fonts are not as crisp and the contrast is lower. It is not by much, but I can notice it (not even without some annoyance)."
This seems to justify iRex not immediately jumping to VizPlex until they can figure out how to optimize it, and keep their advantage via display driving techniques.
In summary, this reviewer really likes it but just not quite enough:
"For me, the CyBook is the second-best e-reader on the market, right after the iLiad -- though it all depends on your needs."