I bought a 600 just as soon as I possible could (get a red one). I love it.
That said, I had an eBookwise first, and I've missed the touch screen and ability to make notes from the first.
My biggest annoyance with it is the screen's reflectiveness. Yep, when I compare it to the 505, the 505 does beat it for bright-and-crispness. But the screen turns are noticable faster. And I've used the dictionary more than I thought I would...which has been kind of fun. (I was always a weird kid - I actually used to read the dictionary when I was bored).
And while others have had different experiences, I find the note taking feature to be gawd-awful slow. And that includes bookmarking.