Since I just got my PRS505 a few months ago, I can still reply (I'm sure in a few months I'll have lost all semblance of control of what e-books I have....)
1) how many e-books have you purchased total (all sources)?
193 purchased total for the Sony reader - but 28 of those are "purchased" with the Sony credit, so I didn't really pay for them. So 165 purchased books in 2.5 months or so... And then I have around 30-40 books for the Palm in my Fictionwise bookshelf, those have been bought over the past couple of years. So the e-book hasn't exactly helped me 'save on books'
2) how many e-books have you purchased via Connect?
I only found 28 remotely worth downloading for the "free money" giveaway
3) What kind of books are you buying?
Scifi, thrillers, mystery, romance
4) How many new books verse e-copies of books you already have?
Some of the ebooks I've bought I already have in paperbooks. Maybe around 15 or so.
5) How many backlist (i.e. older) books do you have, and by who?
I'mnot sure I understand that question... I buy books that I enjoy reading regardless of when they were published when I find them.