It's amazing around here to see a bike with any lights at all. seems to be a local pastime to set the kids a poor example and have no lights, and use the pavements.
That said, they can be noticeable if the flash rate is fast. In fact they can be distracting. But if the period is too slow I reckon one could lok at a junction, not see the bike because the light is off, look the other way, again see nothing, move out and WHAM !
I'm unsure if motorcyclist can use them as a replacement, but I'd have thought they could use them at the same time as a normal headlight. After all the light is also for you to see ahead with, not just to be seen, so a normal headlight just seems common sense to me.