I think the intent of the amber light is that it's quite different from reflected sunlight and other clutter like that. It's less bright than white, yes, but it's still as bright as a car flasher or construction beacon, and you can see those well enough in the daytime.
I've got one, but don't use it much because 1) I'd rather have just one light on the bars, and 2) I usually ride my dynohub-equipped bike when I'm just tooling around town.
It's legally ambiguous, though, as laws regarding bike lights only refer to white in the front and red to the rear. But, I don't think that legality should be a problem.