Almost every comics app—comiXology's Comics, iVerse's Comics+, Graphic.ly, and every publisher app—works the same way: The app itself is free, but you have to pay for the comics (well, most of them).
ComicZeal is the opposite: The app costs $8.99, but with it you get access to a huge amount of free content. I don't think the app provides much that you couldn't get for free* if you put the pieces together yourself, but it bundles everything together nicely and makes it easy to use.
ComicZeal reads PDFs (and RARs) and the file types that are most popular for downloadable comics, CBZ and CBR. It's more of a reader than a store; the big attraction, for a lot of people, is that they can simply import their existing comics library into it.
Wait, you may be saying, where do you get that existing comics library? Ah. Some people scan in their print comics, which is more trouble than I'm likely to ever go through. This guy bought DVDs of Archie comics and imported them into iBooks; you could do the same with ComicZeal. You can buy comics in PDF form from DriveThruComics, and they have some free offerings, too. CBZ and CBR are popular formats for bootleg downloads, of course, but we will not speak of this. Because to my mind, the highest and best use of ComicZeal is to read public domain comics from the good old days. ComicZeal syncs to two sites that download public-domain comics, Flashback Universe and Golden Age Comics. These sites provide a cornucopia of forgotten comics: Romance comics, space comics, detective comics, Dean Martin and Jerry Lewis comics, all the treasures of a misspent childhood.
All the comics on these two sites are in the public domain, and they are available for free download. I could get a CBR or CBZ reader, download the files to my computer, and read them on that screen, but what you pay for with the iPad is ease of use, and ComicZeal delivers: The comics are arranged in a storefront format, more easily browsable than the original sites, and they download to my device with a single touch. ComicZeal has a nice display menu that shows a longbox icon for series and cover images for singles.
There are a lot of different ways to use ComicZeal, and people have lots of opinions on its usability for different functions, but to me it works great as a gateway to vintage comics. These aren't great works, for the most part; they are schlocky genre comics, but that's exactly the sort of thing that doesn't get collected and reproduced. The public-domain download sites are doing us all a great service by putting them up, and ComicZeal streamlines the download process and makes them easy to read. I would cheerfully shell out $25 or more for a collection of these oddball comics in book form; looked at that way, $8.99 for the app is a bargain.
*CloudReaders is a free e-reader that can handle CBZ files. I haven't tried it, but it looks like it would be fine for reading comics files you already have; what it lacks is the storefront of ComicZeal.