Sometimes a picture really is worth at least a thousand words. I'm going to tell you why I think Salzmann Spoke Reflectors are one of the best safety accessories you can buy for your bike, but the above photo quickly makes the point.
Salzmann Spoke Reflectors are 36 little plastic tubes covered in 3M Scotchlite material. The tubes are 70 mm long, slit along the side and shaped so they easily snap securely onto a standard bicycle spoke. It took me all of two minutes to install Salzmann Spoke Reflectors on my Allant. My Allant has 36 spoke wheels so I placed a reflector on every other spoke. The Salzmann folks sent me two sets of the reflectors for this review but these little reflectors don't need to be on every spoke.
Compared to the CSPC wheel reflectors your bike may have come with, Salzmann Spoke Reflectors are basically better in every way. They're very light and what little weight they have is evenly distributed around the wheel. They don't look dorky, in daylight you barely notice that they're there.
But at night, they shine. Really shine. They're reflectors, so they provide no light of their own, of course, but they are highly reflective and glow like bright moonlight reflecting car headlights, streetlights, lights from store windows or other bike lights. They reflect light back from every angle and the spinning movement of your bike wheels really makes the reflected light visible in the darkness in a shape that clearly says "bicycle."
Salzmann Spoke Reflectors are not a replacement for good front and rear lights, but they are an inexpensive and effective way to greatly increase your visibility. At less than $20, they're a bargain.