I have the Kindle DX, (going back) and a Sony 505 and Irex Iliad.
Very disappointed with Amazon on the DX. Upgrade to 2.3 was good, and now PDF's do reflow,
but no folders or tags. Recent post from Amazon that they will address this in first half of 2010... So there you go. Also, speed and contrast on Sony 505 and 300 are much better and tags, folders, and table of content.
The Kindle wireless is nice, but after 50 docs on the thing, it is a mess to page through with no tags or folders. Imagine comments on their thread by users with 200+ books and PDF.