As a Kindle DX owner, it's all about the form factor and readability.

Back lit screens cause eyestrain for those of us who read quite a bit. With my DX I can read for hours on end in any environment I could read a dead tree version in. And with the battery life measured in page turns, even when reading at a crazy clip I can go more than a week without charging.

Form factor was the other huge sell. If I want the ability to read my collection of technical PDFs without a microscope (or awkward panning) the large size is needed. Even with non-PDFs I like the hardback size font and character count on lines.

The DX is the only game in town to get both of those right.