I used to use it to calibrate the screen, but it wasn't very useful because it relies you the user to calibrate by sight. Your eyes adjusts under different lighting conditions, so the adjustments were off most of the time.
I found calibrating the screen to an existing print photo to be better. Just try to match it as close as possible.
But I have since bought a Spyder2. Nowadays, I let the machine do the work.
unless you are confident that your eye is good enough to tell of any colour shift, try not to use this...if you are not particular with colour, you can try it...but then if you are not particular with colour then you might not even want to try it...
if you are particular with colour but do not want to invest in hardware, you can hire someone to do it for you...there are some guys in CS providing this service