I'd support what HarryT has said, though I don't seem to get quite as much life out of the Cybook battery as he does (maybe if I spent less time looking up all those words I don't understand....).
If anything, the navigation is nicer/faster on the Cybook if you have multiple books (you can configure how many books to display in a screen full, up to, I think, 25 -- I don't have the Cybook with me at the moment -- and you use the up/down/left/right to get to the book you want). Whilst I love the iliad's interface, wacom tablet and flipbar, the Cybook seems faster to get to books.
That said, I've not had that many books on the Cybook -- I try to keep the numbers down to one or two screenfuls, moving books off once I've read them, and on when they're "about" to be read.
One point other people mention is the fragility of the Cybook. I've not (yet
) had any problems, and I suspect that if you treat it gently and keep it in its case you should be safe with it. But that might be something to bear in mind.