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.
Halftone 2 is free, but as a free app, it is limited. It seems at every turn, I was offered a chance to buy more graphic elements. If you buy the essentials pack, it totals US$6.99. I would like to have seen more in the free app, and I think $6.99 is a bit too high for the complete add-on. Most other free photo apps give more features up front.
That's not to say you can't make some very presentable comics with the free version, and in fact I did so. The app is responsive, and the output looks very good. You can share your creations by mail and the usual social services. The app also supports Dropbox and you can print your comics using AirPrint with a compatible printer.
There are other similar apps around. One very well-reviewed app is ComicBook!, which as it happens is free today only. It also has a lot of in-app purchase options, so free will only get you so far.
I like Halftone 2. It works well. I just would like to see the in-app toll lowered so more people could enjoy it. Halftone 2 requires iOS 6 or later.