They've gotten a lot of attention in research studies. Results show that they're fairly effective in improving the strength and condition of muscles at the level of individual muscle tissue cells, but that they do not aid in burning calories.
That's pretty logical. Muscle tissue cells burn calories by converting nutritients into the energy needed to flex and contract the physical structure of the cells. Electro stim presents a substitute form of energy to cause the same basic mechanical activity in the muscle cells. Therefore the nutrients, and their caloric energy value, are left unused.