As you'd expect from Autodesk, SketchBook Pro has all the swagger of a pro-grade painting program, but with an experience designed for those looking to create art on the iPad. There's a wide range of digital pencils, pens, markers, and airbrushes to choose from, all accessed via a simple but intuitive UI that lets you pin toolbars to the screen for easy access.
It's flexible and fast too, enabling you to work with layers, transparency options, annotations, and advanced blend modes. With Dropbox integration plus the ability to import and export Photoshop-friendly files, it's an ideal iPad art app for working on the move.
With a good stylus, a good digital artist can create a variety of art on the iPad, from a quick sketch to a more involved and detailed digital painting.