We looked at the original Halftone app for iOS a couple of years ago and were impressed. The app has been completely rewritten and is more touch-oriented, supporting vector graphics for smooth insertion of picture elements.
Halftone 2, which is iPad-only and was released today, lets you import pictures and create a real comic book look by adding bubbles, banners and other bold graphic elements to your photos. The app also embeds an image editor, so your photos can be filtered to have a more graphic look. There are also tools for fixing blemishes, and the usual contrast, saturation and brightness sliders. Halftone 2 also adds the ability to work with multiple picture frames, rather than just one image.
I didn't get too fancy with my image creation, but I could see the possibilities. Banners, balloons and text are very nicely rendered. The image editor is robust and there is built-in video help. I found I could explore the app and get things done without the tutorial.