I've been successfully charging my GPS and a Sinoele 10Ah USB-compatible battery from my hub dynamos for some time. I've used a Luxos U light (which has a built-in USB outlet), and a B+M USB-Werk to convert dynamo electricity to USB. On one Cycle Oregon, I charged the Sinoele battery during the day's ride, and used it to charge my phone, GPS and iPad during the night. I'm not 100% sure that I generated as much energy as I used, but I had no trouble keeping everything charged for a week.

Now that the USB-Werk seems to have died (see my previous thread), I'm in the market for a new hub-dynamo to USB converter. My requirements:

1. Can be mounted in my handlebar bag and plugged-in to a connector on the bike, so that I can move it form bike to bike. This would seem to rule-out these very clever (but not very waterproof) designs that put the converter in the steer tube, and the USB plug in the steerer cap. In any case, my Atlantis has a 1" quill stem.
2. Doesn't deplete my retirement account too much.
3. Compatible with the B+M IQ-X headlamp — ideally letting me choose between full charging and full light when there isn't enough energy to provide both.
4. Has a cache battery. This may not be a strict requirement, because it may be possible to use my Sinoele battery as a cache; it seems to allow simultaneous charging and depletion. However, a cache battery would be nice; many devices don't take kindly to their charging current being turned off every time one stops at an intersection.
5. High efficiency!

I've found useful (but slightly dated) information about USB charging here, and about available chargers here, but no clear winner for my requirements. What do you use?