I am building up a new commuter/utility bike, and I've decided to put on a Shimano Alfine generator hub to power front and rear lights.
Lights have not arrived, but I now have the hub and wheel in hand. I was suprised -- generator hub/wheel doesn't weigh all that much. The hub is not a big, giant thing, either.
Can't wait to get it all on the bike (not yet welded up, so it might be awhile) and see how it burns.
I decided to go this route after several winter return rides in which my batteries ran out on me.