There's a lot of talk about the iPad and a lot of comparison of it versus other ereaders. The Houston Chronicle has a good comparison review of the iPad vs. the Kindle for reading books. And there conclusions are solid. The basics of this argument are all based on technology.
The iPad screen looks beautiful. It is colorful and oh so pretty. It is also a back lit screen. The Kindle uses eInk and has a reflective screen. Reading for long stretches of time on a back lit screen can lead to eye strain and Computer Vision Syndrome. An eInk screen is almost like reading paper, which is much less strenuous.
So when it comes down to it eInk displays like the Kindle are much better for reading books. Then again the iPad does more that read books.