I have been playing around with the new iPhoto for iOS app for a couple of days now. It is very, very powerful and yet easy to use. Note: it does not run on the original iPad.
Ars Technica has a thorough review here. Excerpt:
The app has a lot more photo tweaking capability than you might expect from first blush, including fully non-destructive editing that can be selectively undone. It also improves a bit on the standard Photos app’s organization, and greatly enhances sharing options. More importantly, for $4.99 you get a photo editing tool that can, in many ways, out-Photoshop Adobe’s own Photoshop Touch.