I've seen these on a couple of people's bikes and the output is near zero. If you want a novelty toy then there are better on the market, if you want a bike lite there are better on the market.
Press Release Arc'teryx Climb Academy returns to the Lake District: May 25th – 27th
News Lorraine McCall Starts Grahams Round
Lorraine McCall began her attempt today at a continuous, self-powered round of the Grahams, a four-month tour of upland Scotland combining walking and bikepacking. If successful, she'll be the first person that we know of to do it.