We've been having a long talk about that stuff a while ago, me and my shop guy. We're both a bit sceptical about it (for the record he doesn't sell any products featuring it) but he has seen the technology first hand and said it was pretty incredible. For a demo they had a ball D3O, which was very soft, but as soon as you dropped it or punshed it, it got extremely hard he said, it's also being used in motorsports already.
My biggest concern is, does the whole area gets stiff and absorbs the impact or does just a small surrounding near the area of impact hardens? If the latter, then it still wouldn't give you sufficient protection when hitting a rock/ice/branch. It would only/mostly protect you from... damn, don't know the english word... but I mean cuts and scratches and all that (abressions?), but you could still crush your skull with one of those on.