I use Kindle and iBooks on both iPad and iPad mini - both work, both let me read easily and comfortably.
I've got Marvin, tried it and carried on reading in Kindle and iBooks because that's what I wanted - I suppose Marvin is great for the tinkerers as usual but a lot of people have books to read them and never got too bent out of shape because paper books couldn't have the page background changed, fonts tweaked etc... some of us are just happy reading on our eInk readers and tablets... each to their own but don't write off an App until you've tried it, often simple is just great...
