For anything that has pictures, charts, graphs or formulas, I definitely want the dead tree version. I can't imagine using a Kindle for my college textbooks or any technical manuals The zoom features aren't great and you're sometimes reading text on one page pertinent to a graphic on another page. I already regret having bought 'Capital in the 21st Century" as an e-book precisely for that reason.
For pure text, like a novel, the Kindle is awesome. Portable, comfortable to hold, long battery life. If you've ever read a really fat paperback (Mark Bowden's "Blackhawk Down" comes to mind), just holding the damned thing open is a pain. I love the fact that you can read the Kindle, say at breakfast, without having to use hands to hold the book open. Same with a stationary exercise machine. It's also nice that you can zoom on the text in case the motion of your head is making the print hard to read.
I think the best and most invaluable feature of having an e-reader is Project Gutenberg (gutenberg.org). Want to read or reference Nietzsche, Dante, Shakespeare, or go back and read Moby Dick, The Count of Monte Cristo or many other old classics? All free (and legal) via the web site.
Of course there's always the zombie apocalypse scenario where electricity would be hard to come by, but until then, the e-reader provides a lot of utility .