Doesn't matter if reading has "hardly any impact" on battery life, just having the thing on means it's going to die in a number of hours, as opposed to the Kindle's two weeks.
And do you really prefer reading on PDA's over reading an actual book? I just can't get that. I tried it, I read the Hitchiker's Guide to the Galaxy series and Stephen King's Dark Tower series on a PDA with roughly the same size/resolution screen as the iPhone (Tapwave Zodiac), and while it was doable and nice and portable, I was happy to go back to real books. And reading on the Kindle is just like reading a real book, in terms of how the print looks and the size.