The plusses: the Kindle app is good, especially with two-column layout in landscape orientation. Quite lovely, really. Annotating is much quicker and easier than on the Kindle (though productive of those annoying fingerprints). Also, it's nice sometimes to read without a lamp on.
Summary judgment: while I'll probably be doing most of my web reading on the iPad, any long-form reading I'll save for the Kindle. And for actual codex books.