I have been using a Sony Reader for a few years now (505 model) and have read numerous books and articles on it. I have read for hours at a time, days in a row, all without charging it. The screen and the battery are truly compelling arguments for the eInk technology. That said, I have also read several books and articles on my iPhone using the Kindle reader, the Eucalyptus reader (Project Gutenberg books), the Classics app, Stanza, and even the Barnes & Noble reader (by far the worst reader I've tried). Given what I've seen with the color screen and the smooth capacitive touch controls, a 9.7" iPad will also be a very compelling reading device. Say what you will about reading for long stretches looking at an LCD screen, or that 10 hours is a weak compromise agains the weeks with eInk. The fact remains, if the iPhone is at all tolerable as a reader (which is a very defensible position given the popularity of such reading apps on the device), then the iPad will be at least as good. And given that the most common complaints I have against the iPhone are the battery life and screen size and that the iPad improves on both fronts, then the iPad should be a better reader than the iPhone.
I for one am eagerly anticipating getting an iPad that runs a fully optimized, iPad-sized Kindle app, or Eucalyptus app, or Classics app. That plus the iBooks app/store will pretty much kill off any need or want I might have for my old Sony reader.