Apple has released a list of the top apps for iPhone and iPad in its iTunes store, and three of the top five grossing book apps for the iPad are not just comics readers, they are all from comiXology: Marvel Comics, Comics (their multi-publisher reader), and DC Comics. This reflects not just the quality of the iPad as a comics medium for comics but also the large numbers of comics that must be selling through those apps (the apps themselves are free). The top grosser in the book category is The Elements, a visual exploration of the periodic table, which probably doesn't have a lot of mass appeal but sells for $13.99, and the number five app is The Cat in the Hat, which does have a lot of appeal and sells for $3.99. That three comics apps can match that tells me that people are buying a lot of comics through them.
The pattern is the same for the rest of the top ten book apps—all but the comics apps are single-book apps (as opposed to an e-reader like Stanza), and none are free: Alice in Wonderland, the Bible, a Toy Story read-along, and two more Dr. Seuss books.
The Marvel and DC apps are number three and six, respectively, on the list of most downloaded free apps.
Comics by comiXology is also the top-grossing book app on the iPhone, and the only free one in the top five; the others are a free-book app (pay for the app and the books are free—the opposite of comiXology's model), a free-audiobook app, and two versions of the Bible. None of the comics readers makes it into the top ten most downloaded free apps for the iPhone.
What all this indicates is that comics readers are buying a lot of product through the iPad, and they are buying it from Marvel and DC. That doesn't mean that the non-superhero readers aren't there, but the readership for any one comic (Scott Pilgrim, say) doesn't match the number of Marvel and DC fans.
Missing from all these lists is iVerse's Comics+ reader, which has a smaller selection of Marvel comics. It is also slower and a bit harder to use than the comiXology apps, although I suspect readers are making their selection based on the comics available rather than ease of use. Panelfly, which started as the artier of the iPhone comics readers, is also not on the list and doesn't have an iPad app at all. Graphic.ly, available for iPad and iPhone, got a late start this year and is still building an audience; it does carry Marvel, but again, the selection is small.