I think Zbrush can be used as a powerfull painting tool for concept art, since it combines smart 2d-2.5d brushes, with 3D props and materials, all in one place.
In this step by step walkthrough I explore a workflow that can be used for quick, concept art or even for finished matt paintings (with a bit of help from old PS).
Before I start with the steps, I have here 3 zscripts showing you the ways I use some of the 2D tools:
The first Zscript shows you how to create a blocky alpha brush, for expressive sketching and painting.
The second Zscript shows the process I use for sketching in perspective.
The third Zscript shows you how to use the Highlighter brush to create light, light beams and atmospheric conditions.
In the step by step process, I take for granted that you know the staff that I show in the Zscripts. If you don’t, then download and play them.