I have both Sony's
e-reader and Bookeen's Cybook. My preliminary impressions
are:

Sony e-Reader	Bookeen Cybook
supports PDF, RTF, text?	supports PDF, no RTF, text, HTML - a big plus for me
9 ounces	6 ounces and it makes it noticeably lighter
small, medium, and large; supports a limited number of fonts actually supported	choose the font size you want; lots of choices the guide talks about a font directory to which TTF can be added but I didn't find one and when I created my own it didn't work
has a sleep mode and starts back up pretty quickly	does not appear to have a sleep mode; does a full restart every time
easy navigation, I particularly like the way they use the number keys to let you go to a specific page	the main problem is with the navigation button which requires a VERY firm press but the menu system is nice
handles both SD card and memory stick	SD card slot but I haven't figured out how to make it work
wipes the magnetic strips on bank cards	non-magnetic

but,
when it comes down to it, I prefer the Books.app on the iPhone.
Yes, it would be nice if it had a bigger display but the
display
it has is clear and readable even the dim light of a theatre waiting
for the movie to start. I've read Lessig's Open Culture and
Moon's SheepFarmer's
Daughter and it was fine - the scrolling is much more
responsive than the paging functions on the e-ink readers.