I have a different setup than most but figured I'd post what I do for rear light. I mount it to the top of my rack, but only because I don't have anything convenient to mount it otherwise. It sits in a bit from the end of the panniers but the field of vision is still very large. I will correct this at some point (maybe this weekend if I can remember).

It's brighter than all get out, the picture doesn't do it justice. The pattern on the ground is moderately sized, perhaps a 6 foot circle of bright color. I should probably go check out the latest light technology today. It's been a long time since I had to buy anything new. I've never felt a strong need for redundancy (shortish commute) but I should at least grab a $15 something.