Despite the App Store having thousands of applications to choose from I only use about 5. Brushes is a popular painting application for the iPhone and iPad, which I have been using for some time as a portable sketch book.
Besides the intuitive user interface to select color and brushes, Brushes has a unique feature which I have come to love, playback. A video of each sketch can be downloaded along with a high-res version of the image. As a studio artist I find it invaluable to be able to go back and watch how a sketch is composed over time.