E-ink screens work very well, but the Kindle DX has some issues with some types of PDFs. If there is TONS of little text crammed onto on large page (some electronic textbooks are guilty of this) the DX can choke and give a nearly unreadable page. The other thing with the Kindle DX is you are literally within a few dollars of an Android Honeycomb tablet, which offer MUCH more functional PDF reading experiences. Of course the trade-off is less readability outdoors with a much shorter battery life. It's a toss-up, but a Toshiba thrive 16gb is 399, only 20 dollars more than the DX at 379. The strength of the smaller Kindle is readability, long battery life, low cost. The Kindle DX is a pretty strange duck that doesn't quite fit in with anything.
On a sidenote, the Android tablet is more versatile. There are tons of VERY useful academic apps on Android like Wolfram Alpha, something the Kindle DX does not offer.
Sidenote: The eyestrain thing is a little subjective in my opinion, I only notice it on a tablet if I am in a very bright area. If its between a Kindle with a lamp or my tablet, its pretty much the same for me.
Main system: Core i7 3770k @ 4.5ghz, Gigabyte GA-Z77X-UD5H-WB, 32gb Corsair DDR3 1600 RAM, EVGA NVIDIA Geforce GTX 970x2,
Tablet: Surface 3
Phone: Nexus 6