Bluefire Reader is a wildly popular iOS app that supports Epub and PDF. Unlike Stanze, Bluefire reader supports Adobe DE DRM. It works with any number of ebookstores and it even supports library ebooks. It has long since replaced Stanza as the touchstone reading app for iOS, and I would say that Bluefire Reader might give Aldiko, the leading Android reading app, a run for its money.
Bluefire has some way to go before it will be as good as Alkido, CoolReader or Moon+. First off when it installs it takes over the ePub file type as the default without asking the user so minus one point for the arrogance factor.
The "A"-factor would be moot if the app was appreciably better than the competitors. Its slower than Aldiko, which makes it the slowest of the top reading apps. After changing font size then leaving the app and jumping back the word wrap got ruined so that the text ran off the screen and some fiddling was needed to force a manual reformat.
The only way to load books besides "buying" them from the 3 stores is to copy the files to the Bluefire/Import folder. It would have been smart to make this redirectable by the user to point to the common pool of eBooks shared by other apps.
The last big problem I had with Bluefire was not being able to point it to my Calibre library on my desktop.
The note-taking/bookmarking is Bluefires strongest suit, with the text selection being the easiest to use among the top readers. Unfortunately its a bit too easy--you invariably begin selecting a region if you are too casual when tapping to change pages. Note taking is also exclusively internal, unlike the other readers that tap into Evernote and blogging or note taking apps.
Overall I think its a robust reader but I'd wait for the next version for a speed increase, better library management and more features. I am not too optimistic about an iOS-centric app getting much development time.