My 600 is on eBay - I stole the 505 back from my wife who hadn't yet formed an attachment.
I think the light conditions I read in are lower than others. I loved everything about the 600 except the display, but for me that is the most important thing. I find the 505 display superbly crisp with great contrast and virtually no reflection.
I don't regret buying the 600 and having the chance to play with the new features. And I really hope that the next Sony model in the 6" class is back to the clarity of the 505 - I'd buy it like a shot. I worry that the slew of new technologies, and pressure to include video and colour, will mean that the 505 remains the pinnacle of reading technology for some time to come
amjb