I bought the DX for the large screen so I can read programming books and pdfs on e ink. I like some of the artwork, esp the one of the fish. I think the kindle 4 smaller devices have more contemporary screen savers.
I didn't anticipate how much I would come to prefer my DX to everything else. I do wish that screensaver (that isn't quite the right term) was at least somewhat customizable though.
I bought the DX in order to read PDFs on it. No conversion is necessary - just attach a USB cable and drag-n-drop the PDFs to it. The page switch time is fine.
The only real downside is you can't "flip" through the pages. Reading a book sequentially works fine, any other way is a pain.
I got the DX at the $250 special Amazon had for it last Monday. The price is back to $379.
It works quite well for reading books sequentially, even technical books. One problem I had is that their fixed width font is not the most readable in the world. For jumping back and forth, the experience is not great, but not completely intolerable either.
The DX (and I think also the other Kindles by now) reads pdfs natively, no need for a conversion service.