I've used them. I got some cheap ones from one of those Chinese web sites. They are terrific attention-grabbers. I've even received compliments from them. But mine were not stretchy, and they were uncomfortable. I stopped using them. Maybe I should make my own.

But there are so many accessories involved in commuting, I have limits. Maybe a reflective vest would help, but I just can't bother with some of these things. One thing that has helped a lot is a dynamo-powered headlight. It's theft-resistant, so I leave it on the bike, and I never ask myself if it's time to charge the battery, since there is none.