It may be of interest to some that there are actually two very different kinds of these devices out there. There is a stim unit like what is described above, and what is called a tens unit. Both look dam near identical but are used for very different purposes.

Stims as WM7 stated have been around for a long time, and seem to have the stigma of just another snake oil. I have never used one i wont really comment on it anymore.

Tens units however are used in the treatment of deep muscle damage and atrophy. They run at a very low current, and help to repair muscle fibers. I've used this type of machine many times, and can say that while it doesnt "build muscle" per se. It can aid in the healing process after a brutal training session.

__________________
I don't do this for my family, my friends, women, accolades, pride, or ego. I do it for me and no one else, its just part of who I am.