Here is a sketch that I drew on my iPad using Zen Brush, I was at a railway station yesterday and say this guy waiting for his train and he had a very interesting face to draw.
So this is the sketch in Zen Brush:
Today I brought the Zen Brush sketch you see above, into photoshop and added some colour:
I hope you can see how good it is to sketch with the iPad and then do some post editing to a sketch after.
This allows quick sketches to be done on the iPad and it is a lot of fun!
There are many sketching apps available and all a very good to use and cheap to buy!
I prefer Zen Brush but others are as good, it just comes down to personal preference.