Good point! The stop will never be instant... and you'd better have good grip on your shoes, too.
While I think this product will be pretty neat, I can't see it revolutionizing transportation simply because longboarding requires a good amount of mental energy in comparison to a scooter or bike. But I have to admit, it is seriously fun, and the sensation of gliding is awesome.
This might help you stop the longboard quickly, but will it help you stop quickly (but not too quickly)?