We went touring in France, with a four day camping stint in the middle of the route. I charged up the battery in the unit before departure, and then kept the unit on the handlebars at all times. We only needed to use it a couple of times but it happily supplied enough power to top up the phone. Whether this was because it didn't actually exhaust the 'mains' charge, I couldn't say. Tests done whilst commuting suggest it needs several hours of 'wind' to build up a useful charge from flat.
You need to be moving at a fair clip for it to indicate that it is actually charging - although cycling directly into a Mistral wind at 10mph has the same effect
It has one substantial design flaw - there is a 3 position switch to determine whether the unit turns on two white LEDs, charges from wind power, or is charging a phone. When in 'charge a phone' mode, a red LED comes on, but this is [i]behind [/i]the rubber cap over the USB port, so you can't see it. If you inadvertently catch the switch when mounting the unit in the handlebar clip, the LED happily drains away the power in the battery until completely flat.
It can also make a somewhat annoying plastic rattling noise when on the bike - the fan itself feels like it is mounted on a loose axle and this is what rattles.
Forget "drag" - it's mounted directly in front of you, so you would have provided the drag anyway. I was carrying 4 panniers and a tent, so the extra weight and drag was utterly negligible.
Hope this helps. For me it was worth a punt for the tenner it cost, and, whilst not required in an emergency situation, seemed useful.
