As far as a drawing tool the iPad is not unlike its precursor the Etch-A- Sketch.
The advantage of the iPad is that it can actually be used like Etch-A-Sketch still.
It simulates the up-down left-right knobs quite convincingly.
To demonstrate this state of the art device I've drawn the dry stone rubble helix which I built two summers ago.
When I want to erase what I've drawn i just shake the iPad vigorously upside down.