Had Soma's on my cross bike until it was stolen. They're were ok,but remember that your body will block the right one unless a car is directly behind you. Def auxiliary and not main lights. I chose the Soma's because they used regular AAA batteries which are cheaper than button cells and last longer.

Had a set of Knogg's that had both white front and red rear on a flatbar bike. They were ok(see above,the front's were actually useful),but died in a rain storm.