Just curious...why not use iBooks? It reads books in both PDF and ePub...?
To be honest? I'm lazy!
I use a program called Calibre to maintain my e-book library (1500+ books). The library is stored on Dropbox (in my public folder). Then, I use another program called Calibre2OPDS to create a catalog of my books.
Before I discovered MegaReader (and the free app eBook Search by the same company), I'd have to browse to the catalog in my Dropbox via Safari and search for a book. Then, I'd have to use "Open In..." to select an e-reader app to use. It was just a pain to use (and, remember, I'm lazy!).
With MegaReader, I can hook my catalog up right into the app, browse to the e-book I want and have it download/open right there in MegaReader. Simple, quick and painless. Since I don't keep my books on the iPad after I've read them and I truly don't care about too many bells & whistles (I just need the book's text), this works for me.
Whew! Long explanation! But, that's why.
Marilyn