I bought the Kindle DX and was actually disappointed that it didn't serve ads. Instead, it puts ugly pictures of dead authors as the screensaver.
(Also, I'd be fired if I sent internal pdf documents off for conversion... :-)
The DX (and I think also the other Kindles by now) reads pdfs natively, no need for a conversion service.
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.