Haven't used them. I don't think thebenefits outweight the negatives (for my style). I think they would mess up the centre of balance and considering mine is already high I don't need more height. I think Gary Fischer started this and it is a good idea for the right style.
I barely know what cyclocross is. I have never seen a cyclocross rider. When I get back to Ontario on vacation I will ask some old bike buds there. So I don't know about that last part.