I can't think of a single negative for my Sony 505. It fits me comfortably for one-handed reading, (either hand,) has wonderful contrast and clarity, and displays the books I want to read beautifully. The size of the unit is just right for my jacket pocket or the outside pocket of my shoulder bag, there aren't millions of buttons all over it taking up screen room, the page turns are more than fast enough for me, and I get a full two weeks of reading from a charge-up.
I wouldn't care for it so much if I needed a dictionary when I read, or it I wanted to annotate. But I don't do or need those. I wouldn't have bought it to read magazines or technical pdf's with graphics. It's not made for those. I can read text pdfs with no problem, and I don't find other languages to cause problems for me.
All in all... I haven't seen anything that I'd prefer to have.
Stitchawl