I know we were talking in the context of night time, but if you meant daytime, I disagree.
In towns or cities, HiViz works well during the day. On my motorcycle I get SMIDSY'd a lot more than on the bicycle, and when I put on HiViz the number of SMIDSY's is reduced significantly. Note, my m/c has lights on permanently - there is no off switch.
As a driver and cyclist around cities, again during the day, I have a few times not noticed a cyclist hidden behind car until the last second even though I am very used to looking for cycles. Hi Viz really helps here as it can be seen through windows, and sometimes just a glimpse of an elbow or shoulder in hi-viz is enough to get seen. It is about breaking up the outline of the car and standing out against that object, not other cyclists.
At night though, I agree, the colour doesn't work.