Not rucksack light related but now we are in the darker half of the year I can't help noticing the general poor or non-existent level of lighting employed by some riders. Just on my last commute I encountered a rider who had a matching pair of lights mounted on his handlebars, this is good, except they were so dim that even when quite close up they looked like little more than a distant light reflecting off some chrome!
In total contrast, several times I have encountered a rider/commuter in the wee small hours that has a bit of a light fetish going on. He has a front and rear helmet light, two good bright lights mounted on his handle bars, a rear light on the seat post, a rear light on his rucksack, green flashing LEDs around his elbows and red flashing LRDs around his knees! It makes me feel quite Christmassy every time I see him (And you definitely see him!) I can't help but think that this is slight overkill and I for one couldn't be bothered with the faff of setting up all those lights everytime I headed out on the bike.....