I've got a sony prs-505 and a Pocketbook 902. At home, I only use the Pocketbook. Although both screens are vizplex, I find the Pocketbook's clearer, maybe because it's bigger. I love that I can change the font on the fly without hacking or flashing the device - this has become my top must-have on a reader. Also, the Pockebook can read lots of formats, so I can buy almost any book and don't have the hassle of converting. It reads most pdfs pretty well, this is handy as I get a lot of them at work.
My prs-505 is my on the move device. It's small, light and very pretty. Plus, it has lots of buttons, and I love buttons, so much more convenient than a joystick. Incise: why are all the manufacturers so afraid of buttons? Even if touch screens become the norm, I'd like to have the choice to press or touch
I have changed its main font and installed prsplus. The Prsplus interface is, in my opinion, far better than the Pocketbook's, when I use my Sony for a while I always have a hard time coming back to the Pocketbook. Drawbacks? The screen is a bit dark for my taste, although it behaves beautifully in the sun. I can't change the font on the fly, epubs are not justified and pdfs are difficult to read. But I agree with Elfwerck: 'I want a 5" screen version of the 505. (And I don't mean a 300 or 350. I like the memory card, and I don't want a touchscreen.)'