Who says everyone must be running iOS 6? I still have iOS 5 on my iPhone precisely so that I can run Stanza on it without any issues, until Marvin for iPhone comes out. Besides, Stanza isn't totally dysfunctional on iOS 6, just buggy (crashes in some scenarios). No matter how weird that sounds, but a buggy Stanza is still ten thousand times more preferable than flawlessly working, but utterly lame apps like iBooks and the Kindle app.
(Of course, it gets worse if you have iPhone 5 like the OP, because then Stanza doesn't utilise all of the availble screen and in that case, Bluefire Reader might indeed be the best option to use until Marvin for iPhone comes out.)