Why do you want a Kindle DX? Reading PDFs in the sunlight? I've always wonder who's buying the DX, because at price, you might as well get an iPad.
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.
As a developer I too spend all day reading and writing text on a screen and I can can tell you that there are so many things that go into what causes me eyestrain personally that I'm sure making any of it objective would be tough.
I have tons of problems with eyestrain when reading black-on-white text for long periods of time, so the kindle works great for me. I'm sure with a sufficiently configurable reading app I could tweak it until I could stand it, but I don't know too many PDF readers that allow you to start messing with background colors and the like.
As for Kindle DX vs regular Kindle - I don't have the best eyes so I usually prefer bigger screens. I have a Droid X rather than an iPhone or another Droid because the screen is easier for me to use. I'm also very tall and have large hands. Bigger products are usually more comfortable for me to use.
There are legitimate reasons to chose the DX over the iPad and other Kindles, too. PDF's is a big reason to chose it over smaller Kindles; the reading experience of e-Ink, battery life and light weight are reasons to chose it over the iPad.