O, it's been a long and exhausting process to get things done.
Basically I think it's pretty fun to draw on an iPad, but since the touch screen just doesn't have any pressure sensory, you won't have any brush size/opacity variation automatically while drawing. It was pretty frustrating at first, but after a while, till I adjusted to the way of configuring the color and opacity (and some other properties) manually, I found it was quite fascinating then.
When I first got the iPad, I couldn't wait to try out the drawing apps, but found it was pretty difficult to figure out how to really draw something seriously. Until I found a video on Youtube which was an artist from New York, named David Kassan, doing a portrait for a man with his iPad. The way he did the draft and build up the color foundation etc, was a great inspiration, and I realized quit sketching would make things a lot easier. So I tried the method for a while, and finally adjusted the way of drawing on an iPad. It takes quite a while though, but it totally worth it, I learned a lot from it anyway.
Well, I actually got both Brushes and SketchBook Pro installed, which were two most popular drawing apps on iPad. I did this finger painting with Brushes, for me it's a lot easier to handle compares to SketchBook Pro, not that SketchBook Pro wasn't good, but it just took some more time to learn than to draw instinctively like with Brushes.
Oops, forgot to include this action video, you will see how many times that I modified the left side of the face, it was so frustrating that the finger will block the object you draw, thus it's hard to see what exactly is going on under the finger tip