In regards to the iPhone, where I can compare my use of Instapaper, the [Kindle’s] screen really makes a big difference for extended reading. But the biggest advantage is the Kindle’s lack of any good use other than reading. When I pick up a Kindle, I read.
— 5typos.net: My Kindle 2 review (before the reality distortion field)
This is one reason why I love reading on the Kindle, and one reason why reading (with or without Instapaper) works so much better on anything else, including the iPhone and hopefully tomorrow’s thing, than on a general-purpose computer: the setting and flow of computer use don’t force (or, usually, permit) the focus needed for meaningful reading.