I've been taking advantage of my unscheduled time this week to do a bit of practice with my Bamboo tablet. It's a good thing to take up between activities, since it requires a different sort of focus, and it's easy to slip into "flow" while doing it.
The most challenging thing about using a tablet is that what you are drawing and where you are drawing it are physically distant. If you look down to see what your hand is doing with the pen, you see motion only, not line. If you look up at the screen to see the line, you lose the immediate feedback that comes with watching the movement and line meet at the same point on the paper. Shading and coloring I have found fairly straightforward; drawing and sketching are tricky. Handwriting is next to impossible, except in the most simple, childish way. (I can see why webcomic artists prefer to use fonts based on their handwriting.)
I am getting better, and have discovered that contour drawing - an exercise where you follow the contours of an object with your eyes while tracing on a piece of paper - works in subtle ways to improve my hand-eye-screen-object coordination. This confirms for me that something that would be very useful would be a guidebook that is aimed at the digital artist in a way that current books on digital art and conventional art do not. The digital art books are all focused on how to manipulate the programs; they are not about training the artist's hands and eye and mind. The conventional art books, conversely, are very tactile and visual in their exercises, but their exercises translate poorly to the drastically simple world of tablet, pen, and screen.
In the gap between them is the interaction of physical body with virtual space, an interface that seems to be of little interest to either. But digital art is not just an abstract virtual activity - as the legions of webcomic artists who have to yield to illness or injury well know. Pretending that it exists in a space without bodies does no one any service, and it creates an artificial distance between digital art and conventional art. Both, in the end, are about an artist using mind, body, tools, and imagination to create something, to capture a bit of what it means to exist in the world.
I wish I were a better artist, because it would interesting to write (and illustrate) such a book.