Amazon is trying very hard to be to ebooks what Google is to search today. This acquisition improves their chances of building a monopoly, but the ebook market (reader hardware & software, ebook formats, DRM, publishing, retailing etc.) is still very immature and fluid. So it's too early to declare winners, though Amazon is clearly leading. But all the activity makes the ebook space very exciting to witness and be part of right now.
I love Stanza. I'm just finishing Candide on my iPhone right now. It has a great interface, so hopefully Amazon keeps their promise not to mess with it.
Although I have to admit the possibility of a combined app with the Kindle's selection of new books and Stanza's selection of free books is appealing.