Of all the free photo editing apps I have come across for the smartphone and tablets, this one has got to be best*. The interface is so well designed that the learning curve is a snap (pun aside). Plus it is free. The paid upgrade doesn't do much more; so it is just as well.
Granted it does not have a slew of effects (HDR and panorama stitch would have been nice), but it does all the basic things like crop, straighten, rotation, exposure, saturation, tint, B&W, contrast changes. Most of these are done by simply swiping a finger across the display (how well thought out!). It does go further with sketch, soft focus, sharpening, as well as a few effects. I can see Adobe easily adding more tools and effects and the interface would still be just as simple and as uncluttered as before.
Adobe has certainly set another benchmark for what a tablet's photo editing app should be like. It certainly did with Photoshop (for graphics artists), and later with Lightroom (for photographers). I reckon this app should be THE benchmark for happy snappers and smartphone/tablet users.
Check out Luminance too. It costs $0.99. I haven't tried to out but it looks like a great app.