A quick rundown of what I've found regarding EMS and strength gains:
[*]EMS can be an effective way to increase isometric strength as measured on a dynamometer.
[*]Claims of 30% strength gains are wildly exagerated and not supported by the research.
[*]Benefits are highly dependent on both the wave form and the type of machine used (these two factors are highly related).
[*]There is little consensus on whether there are any gains in dynamic strength seen as a result of EMS use.
[*]Benefits appear to be greater on the untrained or very weak.
[*]EMS use may result in hypertrophy, which could indirectly increase strength.
[*]There may be benefits for neural adaptation and motor recruitment but much more research needs to be done.
[*]Gains (isometric or dynamometer torque) do not match those of regular strength training.