There's no shortage of photo editing apps for iOS, and I am constantly amazed at the quality and the creativity of the offerings. Process (US $2.99 universal app) has some unique features that can breathe some new life into your photos. The app features non-destructive editing, so you can apply multiple filters with one layer for each effect. It feels a bit like using Adobe Photoshop on my desktop Mac, but Process is much simpler to use. Photos are saved in full resolution, unlike the somewhat disappointing Adobe Photoshop Touch app for iOS. You can save your effects to use again, or share with others. If you use the app on both an iPhone and an iPad you can transfer the effects between devices via the cloud. The app supports the new Retina Display on the latest iPad.
Process is a pleasure to use. The app has many presets and they are quite good. At first glance, they are not as varied or extreme as some of the other filter apps out there, but frankly, that is what I like about them. There's nothing over the top, and your pictures stay naturalistic, which is my processing style. You can certainly push the effects quite a bit, but they never look like a bad LSD trip.
The app doesn't have some of the more popular effects, like tilt-shift, but again, that's not what this app is about. You can add layer after layer of effects, remove any of them to see how your processing is going, and try different combinations, all without destroying the original image. Help is built-in and is comprehensive.
If you want to do more dramatic filtering and processing, I still like Snapseed from NIK software, but I think Process is very well thought out, extremely flexible, and can improve your images without turning them into Andy Warhol type art. iPhoto is also an excellent choice for photo editing, but it has a higher learning curve.
You can check the gallery for some screenshots.