I always wear my neon yellow vest when I ride. I definitely noticed a difference when I put it on after not wearing it - cars at intersections were much more likely to see me. Personally, I think it is more important than a helmet since being seen means less likely to get hit in the first place. (Helmets don't do much if a car runs over you). The one I wear was made by Canari but is no longer made. It is starting to fade but I am waiting for Pearl Izumi to introduce their new version of the Zephyr vest which will have a back pocket unlike the current one. I even ride my fixed gear bike with this vest on even thought the hipsters shake their heads sadly at my fashion faux paux.

Note to some of the other posters - reflectors aren't very usefull in daylight.