Apparently, the system disengages if you dip the clutch, or press the accelerator, and it won't engage at all if you brake as you would do normally. But if you're distracted and someone suddenly slows... it could be quite useful.
I suspect, like ABS and (soon) ESP, this sort of auto braking (AEBS) will be standard and required on all new cars one day. Anything that reduces accidents, no matter how good a driver you may think you are, has to be a good thing?