The portability of devices like the iPod touch for field work was intriguing but I'd resisted because many lack default pressure-sensitivity and drawing on 3"+ screens seemed awkward. But, a few months ago a hand-me-down kind of forced my (drawing) hand...
Although I hate the glossy screen, which makes viewing challenging, it at least also allows for some anonymity from passersbys, as does just handling a mobile device these days (often a speedbump using sketchbooks or easels). I'd still prefer a matte option though, so that the brightness doesn't have to be cranked up to counter reflections and thereby prolonging battery life too.
After dismissing SketchbookX for feature bloat, I found the streamlined Brushes app. I prefer direct. Some of these studies are finger-paintings, others are done with a DIY stylus and the rest with a $1 stylus. They're from waiting rooms, farmers markets, laundromats, libraries and netcasts.