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].

[1] https://github.com/koreader/koreader

[2] https://github.com/kai771/kindlepdfviewer/tree/librerator

[3] http://www.willus.com/k2pdfopt/

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.

No. That product is years old and has been removed entirely from the market twice now. Amazon deliberately does not market them and only makes them available intermittently.