Here's a nice big (600x800) image of 16 shades of gray.
That should give a better impression than the tiny image you tried. But I'm pretty sure the Cybook only supports 4 colours (that's what their website says) and I've read on E Ink's website that Vizplex supports 8 shades of gray -- but as HarryT mentioned in a different thread the screen controller used in the Cybook
does not.
But does it really matter that much as long as the dithering is good enough? I don't think so ...