My daily use bike has an Alfine dynamo hub. When the light is off, drag is approximately the same as the rear wheel (Alfine 11), that is to say: not much. With the light on, the only time I really notice it sapping power is if I'm already bonking--then I'll flip the light off uphill--but it is otherwise not noticeable. No problems with disconnecting the wheel, though I don't do it much anyway, as I've got tubeless tires on the bike, and I generally make minor truing adjustments with the wheel in the bike.