It does exactly what it's supposed to, though it has some flaws. – Comic Book Viewer Review

It provides the essentials and supports popular comic book formats perfectly well. The app itself does not have ads, which means you can read in peace, not have your screen cluttered, and keep your cell data usage in check. This app is not perfect. As of iOS 9.1, you will have to exit and re-enter the app to get it to load comics correctly. The app also does not support LZMA compression for cbz files as far as I've tested.
Review by Tuxedon Prowess on Comic Book Viewer.