most of my uses of photoshop are to do with precisely that: shopping photos and not all that much more. with perseverance i can produce a map, a layer mask or similar but that's all a far cry from using it as a drawing and painting tool.
i have brushes on my iphone but find that if i fancy a sketch i rather take out the sketchbook than the phone.
but with the harmony surface, and it's limited yet rather complex brush tools, drawing at the computer becomes something different: it's rendering, the shades and lines build up in interesting, and in some of the tools complex manner.
have a look for yourself, here