Modern hub dynamos are a lot easier on the legs than bottles, or the old hubs. Just yesterday I rode in to work with the light off in the morning, and then turned it on coming home. I'd be hard pressed to say it was any more difficult with the light on. My dynamo hub is the Shimano 3N72, a mid-range one. The light is a Cyo premium. I do remember riding with a bottle dynamo on a bike a few times, and it was too hard.