I have one of the old B&M Toplight tail lights, and I like it. I haven't been rear-ended yet, so I don't have any proof that it does a bad job.

I typically run a battery-powered flashing tail light at the same time. On short rides when I'm feeling lazy, I might not carry it. I usually hang it from my tool bag which hangs from my saddle. Here is a picture of my bike with its B&M tail light and my toolbag, tail light setup.