I just got one this weekend, so these are first impressions:
The selection of books is OK. Good for popular fiction, great for classics, and hit or miss for most everything else
It's not possible to listen to an audio book (audible format) and read another book at the same time (it would be possible to listen to an mp3 book while reading). There are some books that I'd like to listen to while reading along, so this would be a nice, if infrequently used, feature.
There's no way to create playlist for mp3's
The screen is easier on my eyes than reading on my Treo.
I would like to be able to book my books into catagories.
I haven't tried yet creating my own "books".
The flash of black on the screen as each page refreshes is becoming less distracting.
The data connection is weak. I have Sprint service for my Treo and almost always have connection to the EVDO network, but the Kindle frequently looses connectivity.
It is possible to use the device without a desktop computer for written books. Audible books require downloading from a desktop. It is possible to download Audible books over the Sprint network as I download them to my Treo often.
There is a significant lag between keystrokes and text entry.
Footnotes for some books are "hyperlinked" which is nice as it isn't easy to navigate back to the middle of a chapter. But not all books have this feature programed.
It's Linux based. I haven't decided yet if I want to keep it as an ebook reader or if I want to hack around with it to see what it can do.