Recently I have been experimenting with the ipad as a painting tool. I don't think there's much chance of it taking over from pencil, charcoal or ink for drawing - it is still not responsive enough to be really useful as a drawing tool - but for painting studies where areas of colour are being massed in, it is really useful. The advantages for me are primarily:
1. It's mobile (until it runs out of batteries) - no need for a whole box of bits and pieces for painting
2. It saves a huge amount of time in mixing paint - you can just select the colour you want, and colour pick anywhere on the canvas.
3. You can use the tablet to photograph a drawing and paint over it as a way of beginning, or photograph a painitng and paint over that in order to try things out.
4. The app that I am using to paint with, "Brushes", has the advantage of recording the process of the painting, stroke by stroke so you can look back at how you approached the process.
I have made a Youtube channel and put a few videos of these stroke by stroke sequences up at:
http://www.youtube.com/channel/UCx8WotSn6QX_W4ECOLu3Oeg?feature=watch