It's hard not to be fascinated with the idea of it, isn't it? The drag is much less now than it was decades ago - a good dynamo is more efficient, the lights are brighter and more efficient.

What's stopped me so far: the setup is more expensive than the Cree LED lights that work great for me, I take the lights off for half the year on one bike at least, and I begrudge even 4-6 watts. And as far as I know, they still aren't as bright as the battery lights. But still, on the commuter workhorse it's a tempting choice.