There has been one study, published in the medical journals regarding the efficacy of these machines. Initially they were used as part of physiotherapy but are now marketed as miracle machines to help lose weight.
The study suggested some difference when the machine was regularly used on the muscles in the abdomen but overall the amount of time taken to put the pads on, sit with the machine on and for the results to show, did not warrant its efficacy.
Concluding that there really can not be any replacement to regular, consistent exercise and a healthy diet as ways to maintain ideal body weight.