I've been using Sony ereaders since the PRS-500. While the PRS-500 was a good ereader, it had room for improvement. As an example, you couldn't go directly to a specific page (you could only go to a percentage of the book or jump by 10 pages forward or back).
The PRS-505 was a significant improvement over the PRS-500. For me the PRS-505 is the best ereader they ever released, and the reason I recently purchased a second one (after my original's battery basically wore out). I do have a PRS-600 and while it has a few nice additional features over the PRS-505, I didn't find it as a good an ereader as the PRS-505 (as an example, the screen wasn't as sharp).
The among reasons I prefer my PRS-505 over more recent ereaders are:
- No touch screen.
- Well-laid out physical buttons
- E-ink screen for easy reading
- Long battery life
- No wi-fi
- An interface specifically designed for ebook reading
I'm very happy with PRS-505 and hope that, when it reaches the end of its usable life, I'm able to find an ereader that works as well for reading ebooks.