At the end of the day, a reader is a reader is a reader and once you actually acquire the book you want, they all do pretty much the same thing. What separates one app from another is what I will call the overall "reading experience". The Kindle app still comes out on top for me because of my comfort level with Amazon. Some people don't like them because you can not buy books from within the app itself like you can with iBooks but it makes no difference to me. As has been beaten to death, the main issue with iBooks, for me, is the fact that you can only read their stuff on an iDevice and not a computer.
Except for the freebies, there is nothing I can't get from Barnes & Noble or Borders (Kobo) that I can't get from Amazon. As far as freebies, Stanza is the "shiznit" primarily due to the fact you can download straight from Project Gutenberg (plus some other free content providers and a few pay sites). If I get really bored, I will look for something I haven't read since high school or college where we had to read a lot of that stuff for English lit classes. I just re-read Bram Stoker's "Dracula" and honestly, it was a lot better than I remember it.
To me, its all about how and where to get stuff to read, because once you have it, all the readers do exactly the same thing.