Every now and then I'll realize I have the light on in daylight, and not the full light but the daytime running lights. So I'll turn it off. Does it make a difference? Nah, I can't tell. But one day I was in Prospect Park at night and since it is a closed road without any obstacles I turned off the fully illuminated light, and yes I could tell the difference. Would it make a difference in my riding, probably not, but I could tell.

I'm using a Shimano hub and Cyo premium light, no taillight.

And in a plug for dynamo lights in general, and my Cyo in particular, people have been commenting on how much light the thing throws off onto the road, even the guys with the light cannons.