I had been thinking about trying a Knog Frog. Colorado Cyclist had them discounted on the same page as the headlight that I was purchasing so I ordered one. They only had blue so I got that by default. I probably would have chosed red.
I used it last night mounted to the back of my helmet. We had a large group and several riders remarked how bright it was and how distinctive the blue light was to get me noticed.
I am thinking about getting a couple of more to give me some side visibility.