I'm beginning to use my iPad as a reader for the assorted books I've scanned myself and converted to image-based PDFs. Still hunting for the perfect reader app that shows two pages side-by-side in landscape mode, but for books where the layout isn't so important it's working nicely.
Waaay heavier than the Kindle, though! I can see each being useful in its own environment.
I'll have to look into how to jailbreak it...