I find these spoke reflectors a much better option than the traditional chunky plastic wheel reflectors. Not only do they provide a larger spread of reflective area when caught in car headlights, but they are much lighter in weight, and therefore won't unbalance the wheel as it rotates. They are also visible over a much wider range of angles than a traditional reflector, because they completely surround each spoke. They can therefore contribute to better forwards and backwards visibility too, which has to be a good thing.
Brilliant idea - makes bike so much more visible from side, and so from cars at junction when you are passing in front of them. Very economical way of much-improving safety!
These really shine bright when car lights get close. They're hardly noticeable in daylight which means my teenage son isn't worried about looking uncool, but I have the peace of mind that he'll be seen when it gets darker.