It's actually quite good if you "root" the device and install koreader[1](the original developer is the same guy who wrote the vnc viewer for kindle, that was mentioned in this thread), or its librerator fork[2]. In contrast to Amazon's practically unusable PDF reader, these open source projects support custom zoom levels, 2-column mode for academic articles etc.
Another option is to use k2pdfopt[3].
Horrible. The screen refresh was fast enough, but the actual PDF rendering was grossly underpowered. I was reading technical articles with embedded plots that would take quite literally minutes to render.
I sold my DX (the "current" gen) and bought a 3rd Gen iPad the minute the retina display was released. I still pine for the eInk display though. I've half considered a hobby hack project that involves a Kindle DX brain transplant.
[1] https://github.com/koreader/koreader
[2] https://github.com/kai771/kindlepdfviewer/tree/librerator
[3] http://www.willus.com/k2pdfopt/