I always feel obligated to defend the Kindle whenever anyone says the iPad is the Kindle killer. It's not even close to being that. The biggest issue is that while the Kindle may be useless at night without a book light, the iPad is useless outside, period. I have to crank the brightness up all the way, and even then it looks like a smeary mess.
Then add the fact that the fonts look too "computerized," and it's too heavy (subjective), I would never use my iPad as a book reader. It simply lacks the warmth of a book with no distractions that the Kindle provides.
Oh, and I can't stand reading text on a computer for any great period of time.
The iPad may well kill the Kindle in terms of what people buy, but in terms of what's better for just plain reading a book, the Kindle positively destroys the iPad.