One of the most crutial aspects of a CNC machine is the motors and control systems used to position the head of the mill on the work surface. Accuracy of these components will of course directly affect the precision and quality of the work done. I have posted a new article on SebbyLive which goes over some of the options (Stepper vs Servo motors) and describes how I made my selection.

http://www.sebbylive.com/projects/hardware/cnc-mill/motor-selection-for-mill-motion.aspx