I've now had the Nano Plus light for a while now and have been able to test its charging abilities fairly well.
It kept my phone charged no problem during my 3 weeks in Ireland, cycling about 4 hours per day on average, I would say an hour of cycling put about 20% charge into the phone with the phone switched on but nothing running.
I was able to continue charging in wet weather (the instructions advise against it) by placing a ziplock bag over the light and usb cable connection and securing it with an elastic band.
Last sunday, over a 3 hour ride at an average speed of 17.3km/h, with phone gps on, screen on permanently, and running a mapping app, the Nano was able to maintain my phone's charge at the same level throughout the ride, and in fact, due to a fast last few miles, the ride ended with the phone battery 10% higher charge than at the start.
As for the light output, I still haven't really been able to assess it, although I did run it with the light on one day when it was wet and dull and on busy roads. I could feel the difference in drag with the light on and freewheeling but couldn't really feel any difference when actually pedalling, when in usb mode I was never able to feel any difference in drag between charging and off.