I have to say that out of all the phone apps, Sketchbook Mobile was the best and one that I might use in the future if I want to fill in a spare moment.
However, phone apps are so fiddly to use and they do take some time to master, that I think if you are in a coffee shop with time to spare pull out a piece of paper and just start doodling and then scan it in later or photograph it. Sometimes there's nothing to beat just drawing with a pencil. My doodles are all more artistic and free flowing/thinking when draw with a pencil rather than trying to churn out something on a phone.
Here is a collage from pictures taken in Thailand to represent the tao of drawing!