Whilst I am not a dentist, or in any way professionally qualified, I can pass on the advice given to me over the years by those who are.
First, how much more effective brushing with an electric brush is than a normal manual one. I have seen this myself in the dramatic difference to how much plaque needs to be removed from my teeth at each visit.
But secondly, and this is important with regard to these brushes, that many dental problems originate from the gum area rather than the teeth themselves. Especially the boundary between gums and teeth, where small particles of food can get lodged and fester away.
My dentists have long recommended that I purchase the softest possible brush heads. There is no need for a stiff bristle with an electric brush, because the action of the brush head is plenty to dislodge anything. However, a hard brush can be counter-productive, because it can deter the user from really getting the brush into the gum line (due to discomfort). Alternatively, it is possible to damage the gum from over-vigorous brushing with a stiff brush.
So, to these brushes. They are lovely and soft, causing no discomfort at all, but still cleaning just as effectively as the 'normal' ones. I would recommend them unreservedly.
The only drawback to softer heads is that it can be hard to find them in supermarkets, which tend to stock only the 'normal' stiffness. Amazon to the rescue again, and at a highly competitive price. Please give them a try - you will not go back to hard old brush heads again!