Minimal effect. Big waste of money.
A better way would be to insert electrodes right into the muscles. That way (assuming you keep the voltage within acceptable parameters) it'll have indeed effect, even more than when doing exercises. It does pose a considerable danger to your health if you use it too long with one muscle-group when you're not in good condition or if you make any mistakes with the electrodes (you can even kill a person with a 9 volt battery), though.
All with all, normal exercises are either more effective or much safer.