The Phoenix Photo Editor joins a crowded field of photo editors for iOS, but it's free and has a wide selection of features.
Of course the app has the usual filters and tools for cropping and saturation. It also adds high-end features like lighting effects, vignettes, selective blur and a nice variety of frames.
One sign of a good app is not needing help when first exploring it, and the Phoenix Photo Editor lets you get right to work without needing pages and pages of explanations. Many photo editors skimp on undo features, and this app lets you go all the way back to where you started, a step at a time.
Metadata is preserved when images are edited, but you can elect to add or edit geolocation information, which is nice. Images do not save in full resolution. On the iPhone 5s, which has a resolution of 3,264 x 2,448, images save at 3,000 x 3,000. Anything with a higher resolution will save at that size.