I got one of those under-BB dynamos at a swap meet and installed it on my spouse's bike. She doesn't plan to ride at night, so it's there in case night happens unexpectedly. (Ooh, that sounds silly, but you get it.) She has ended up using it a few times and says the drag is annoying. It's less than the bottle dynamo but more than a hub. I can see how it would annoy, especially a lightweight rider as she is. Too bad those things didn't work out because they were easier to install than hubs, and seemed like an improvement over the bottles, but yes, they wore out.