I’ve been playing with direct sketching on my iPad recently.
I’ve been using a Wacom Bamboo Stylus as an input device, it really takes some getting used to, particularly as my Wacom Tablet attached to my iMac is pressure sensitive, and of course the iPad isn’t. To achieve any kind of finesse is quite hard too, the pen for the Wacom Tablet has quite a fine point, whereas the Bamboo stylus has a fat rubber point. I think it’s just a matter of leaning to use the kit though.
For software I have been using the mobile version of Sketchbook Pro from Autodesk, this software does a great job of mimicking the desktop version (although I use Sketchbook Designer on the Mac as it also incorporates vector tools). The UI is intuitive with the ability to create unlimited layers, and plenty of undo’s.
you have the choice of many formats for export, but I find expoting a layered PSD (photoshop document) works best for my workflow.