I never used one but I installed a lot of them BITD. They were the cat's whiskers in the mid 80s. Dynohubs essentially did not exist then -- unless you happened to have an old Raleigh.

The main thing is that they could be used with light tires since they didn't screw up the sidewalls. They were a little quieter than bottle generators too, IIRC. By the late 80s they had gone out of fashion as new high tech $$$$ battery systems came into vogue.