I love my original Kindle. But the Achilles Heel of the Kindle has always been technical publications. The original Kindle does not have the screen real estate and formatting engine to properly deal with equations, graphs, and, most importantly for programmers, source code. Although it is possible to view some PDF files on the original Kindle, there are likely to be major formatting goofs.
Enter the Kindle DX, which might as well be called "Kindle: Textbook Edition." The DX features a 9.7-inch diagonal screen (compared to 6-inch on the original and Kindle 2) and can display PDF files natively. It is not nearly as casually portable as the trade-paperback sized Kindles, but it is vastly more totable than a 500-page text.
The eInk display is better for reading than any screen you've ever dealt with before. It's not even in the same league as a laptop display or an iPhone display. My plane-flying routine is to lose myself in reading and I've consumed entire novels on the Kindle during East Coast to Hawaii trips. You can't do that with any other display (even if you had the battery power in your laptop/iPhone etc.).
PDF display is a huge deal because so much technical content comes in PDF form. The Kindle DX displays an entire 8.5" x 11" PDF page on a single screen. Note the lack of a qualifier in that sentence. If you have an 8.5 x 11 PDF page to read, the Kindle DX will display the entire page on a single screen. There is no zoom feature for PDF pages.
I hope these photos make the problem apparent:
This is a fair comparison: the print version of the Communications of the ACM versus its PDF version (unfortunately not the same page, since I haven't yet received the print version in the mail!). The print font is about 40% larger than the display font. It's still readable (for me), but not easily and not for hours on end. Citations, which use an even smaller font, are too small for me to read at arms length.
Now, it's wonderful that the Kindle DX can display a page layout in its intended form and navigating a multi-column PDF in zoom mode is frustrating[, but the option should be there]{style="text-decoration: line-through;"} [Update:]{style="color: #ff0000;"} The excellent comment below led me to enable the 'auto-rotate' ability of the DX (behind the font button). I had locked my DX in portrait mode; in landscape mode, an 8.5 x 11 PDF is much more readable. One could hope that the Kindle could update this with a firmware update, but Amazon seems in no rush to add features to the Kindle (I think it was 14 months between firmware updates for the original).
Other PDF flaws: you cannot attach notes and highlights to specific ranges within a PDF. Although you can add bookmarks, you cannot title them helpfully. So, for instance, in a technical book, it's counterproductive to bookmark within the index; I just drop a bookmark on "M" in the index and page from there.
The DX keyboard has another flaw worth mentioning. For space reasons, they have combined the numeric keys with the top row of letters. Which makes sense but for the fact that jumping to a numeric location is the #1 use of the keyboard! In order to enter a numeric location, you have to hold down an 'alt' key. Interpreting 'QPPP' as location 1,000 is another seemingly-simple feature that one can wish for in an update.
Conclusion:
For reading, I prefer the DX to the original. I read quite quickly and although one eventually tunes out the page turns, it's nicer to have more text on the screen. Also, the DX is significantly better for reading at the gym; on non-PDF files you can zoom the text to a size you can read while on a machine. For portability of technical books, it's hard to express how much better the DX is. For general portability, the original's small size makes it essentially invisible, very easy to tuck into a carry-on, etc.
For a technical professional or student, the ability to view PDFs is a great feature, but is obviously already available on a laptop or desktop. Is the Kindle DX a perfect machine for working with PDFs? No. The lack of zoom and annotations are significant flaws.
Personally, I am happy with my purchase, but I am a huge outlier on the "print consumption" graph -- shipping costs to Hawaii are very significant, I review a lot of technical books, and I subscribe to a dozen magazines. Unless you're in the same odd position, I suggest that you stick with a Kindle 2 for now.