I bought a Kindle 3 recently for exactly this reason, and like it quite much. In hindsight, I should have bought the DX version though. PDFs are shown properly in Kindle 3, but are too small to be read comfortably in full-screen mode; this shouldn't be a problem in the DX. I normally tilt my Kindle 90 degrees and read articles in landscape mode, this works great, but you loose a bit of the overview of tables and figures. I'm currently trying out a software (Calibre) that converts PDF --> .mobi, Kindle's native format, so that the text can be shown with proper size. The text transfers perfectly, but tables and equations are brutally slaughterd.
I should point out that e-readers are NOT REPLACEMENTS for paper copies. You'll want those to scribble notes in, or to quickly flip between the text you're reading and the appendix/table/figure/equation you want to take a peek at. E-readers are great on the go though, so you can have access to heavy books without bringing them along. It's also pretty convenient to search for "where the #&"! did I read that?".
Kindle 3 works great for books btw. In fact it's so great that I've started reading books again, which I haven't done since I was forced to it in high school (6-7 years ago).