I think what MrJ is suggesting is that while they may make you appear more conspicuous, there is little evidence to suggest they make you any safer. The Governments own TRL has reviewed the figures for several different categories of road user and failed to find any link between hi vis and accident rates. The implication of this is that people run over cyclists because they weren't looking, or were driving like prats, and not because they didn't see them.
A somewhat contentious subject, though not as knife wielding and death threat laden as then helmet question!
@User9609 SAR use red, or a mix of hi vis and red, because we and MR were finding that yellow high vis can almost become a form of camo against a grassy or leafy background.