I feel like my drawings got a lot more dimensional and interesting once I was able to achieve variable line width. I love loading different colors into the pen and going HAM on paper that totally can't handle it. My sketchbooks crackle when I turn the pages. They buckle and heave and are exhausted from their tribulations.
Adobe just released a new drawing and painting app Fresco for the iPad. Here are few experimental images. I have to say I am impressed with the natural media brushes and the interface.