For more fine-tuning, when you find an effect you like, you can touch the check mark in the lower-right corner to make the effected image your focal point. Now you'll be able to experiment with a new set of effects in each direction (including Focused, Focused Color, and Focused Contrast) that use your originally chosen effect as the starting point.
When you're finished, the app uses the standard iOS 6 sharing button setup so you can save to your Camera Roll, send through e-mail, post to Twitter or Facebook, and more. But you also have the option to perform further edits in supported apps. On my iPhone, PhotoToaster shows up as an option for export.
Even though I really like this interface for adding effects to images, the app is a little lacking in other areas. With other apps you have the option to add frames and labels, which -- while not totally necessary -- can add a little more style to the overall look. The app also doesn't have a lot of textures, but it does have a crumpled paper effect that looks realistic. I imagine as the app evolves we'll see some of these features added, but for now the effect grid is the main thrust of the app.
With that said, Gridditor is a unique way to play with your photos and the self-explanatory nature of the interface will be easy to understand for just about any user. If you like photo effects and want a new way of jazzing up your images, Gridditor is a good choice.