Paintbook is another one of the dozens of drawing and painting apps that I have added to the iPad. This is so far the only image that I have drawn in Paintbook, which is a vector based program, but it feels more like working with a bitmap program. Because it is vector based then images can be exported at large sizes if required. You can also export PDF files and even multi page PDF documents.
Paintbook has a rather different interface and toolset than many drawing apps, but it is capable of some great results. Well worth a look.