I think at this state it is still bullshit. The electrical stimulation is way too general (water-soaked sponges through the hair, for sure!) and unfocussed to improve so many different diseases lie depression, ADD, etc.
If it works for some, then due to the placebo-effect which is fair enough, but the science behind is not convincing.
If it really would have an effect, it would be much more dangerous to apply and it would not be sold on Amazon.
This kind of stimulation is effective in improving your brain the same way like the TENS is effective in building up you muscels. None at all.
But it fits perfectly well in our convenience-society. Workout without actual motion? Just apply some pads. Improving your brain? Just apply some sponges. It the american way of biohacking.