Sage advice from Wooo; "Crunching is free". And more effective.
Good for increasing a blood supply to a muscle and stimulating nerve responses (I used a device like that after lower back surgery). So the average guy might gain a little benefit from it, but there are limits. For the guy who wants more in the way of max ab work/development, that machine is not going to get you there.
Of course there have been grant studies (most grant studies are always suspect) that suggest any controlled increased blood/stimulus, to the arms for example, can increased their muscle size. Lab electro impulse devices have show potential when used on animals. Volume muscle tissue gains to twice,or more, normal size have been noted. And in a somewhat quick period of time. A future Mr "O" gives credit to machine impulses, high amino diet and 'roids? In theory, maybe very possible. Old BB'ing saying, "More blood, more muscle" and more often. Good Luck.