I recently purchased an iPod 4 with retina display. Allthough the battery doesn't last as long as on my 600 (doesn't even get close), the iPod (for me) is just so much better as it fits in my pant's pockets. For at home, I mostly use the iPad now.
Leaving the 600 more or less on the shelf, just because the apps are written by people who seem to be real readers who implement stuff they find useful in the first place, rather than having engineers just doing a job for Sony (or any other company).
I'll probably switch back to my 600 as soon as it gets warmer for reading outside as the display works much better than the lcds in bright sunlight
