I use the Kindle app, borders app, stanza, and kobo. Mostly to take advantage of specials
The Borders app can be buggy (I didn't notice this on the Kobo app, which I only used a little bit), but it's brilliant when I go to the store and search for a book. One time I was looking for a book at Borders, did a search on their computer, and learned the ebook was on sale for super cheap. I was able to log in to my account on my computer, order the book and send it directly to my iPhone. That was cool