Cameron, I have a Cintiq 24HD and have tried using it for CAD in Rhino, I have not found it to be useful for my workflow. I tend to be selecting points on lines and manipulating views in my workflow. The energy and the amount of linear distance that my hand has to cover is an order of magnitude higher than the mouse.
For pin point selection I find the small glass thickness parallax gap between the mouse pointer and the pen tip to be irksome, and well as my hand covering the screen.
Right hand on a super precise, optical mouse pad surface, acceleration sensitive mouse moves in about two square centimeters of space, that seems really hard to beat.
Left hand typing shortcuts on keyboard and moving to Spacepilot type devices for view manipulation.
Distance between my two hands is normal, slightly wider than shoulder width, equal elbow angles, in the same plane, horizontal table surface.
I am curious (seriously, not sarcastically) as to the workflow that I might be missing. (and thereby make my investment return better on the Cintiq!) Possibly a video of working on CAD on a Cintiq that you can recommend?