Pneumatics works best when you need two-position linear motion with no stopping between the extremes (extending or retracting an arm, for example). You can vary the actuation speed using the regulator valves provided in the kit of part. When you need multiple random steps within the range of motion, motors and pulleys work best, although (as has already been said) there are ways to achieve this using pneumatics, but they use a LOT of air. Check out this discussion:
http://www.chiefdelphi.com/forums/sh...ad.php?t=23934
and this white paper:
http://www.chiefdelphi.com/forums/p...ion=DESC&page=3
This is only our second year, so we are still not comfortable with pneumatics. Also, our primary sponsor is Intel, so our mentors are primarily electrical engineers; they like motors.
