As a Sales Rep doing a lot of miles in both urban and rural envioroments, just chucking my tupence worth in as well.
I think yellow Hi Viz is vastly over rated as a safety tool in the busy urban envoiroment for cyclist to think it's going to make them stand out anymore. There are just so many 'others' also wearing hi viz for the cyclist to be the exception. Everyone from workmen to binmen, charity collectors to paramedics seem to wear yellow hi viz, the cyclist wearing hi vis just another jacket. Okay, it may still help, but it isn't a magic defence shield.
In less busy area I do think it serves it's purpose, in rural ones, maybe but I do tend to sway to the blend in point of view.
My big exception, at night. The yellow bit is useless, but the reflective parts works a treat and draws your eye, paticularly of movement is involved. However, and this is a big however, don't wear a rucksack over top and block off the said reflective strips. Ditto with back lights pointing to the stars or down to the ground. Point them horizontal so they can do thier job.