I've been using the iPad to edit photos for a couple years now. I still use my MacBook Pro for the majority of my processing, but some photos are fun to work with on the iPad. Especially those that were shot with my iPhone. My go to app is Camera+. The iPad version of the app has a number of features not available on the iPhone version, including the ability to use multiple effects. The iPad app also allows editors to paint in or erase out the effects on specific areas of the image. The iPhone version only allows percentage adjustments to the entire image.
The two photos below were taken with the iPhone while out walking the dog this evening. I transferred the images to the iPad for editing.