Derailed

Have you considered the E-Werk? I have mine now, and I'm very impressed with it. It is smaller than I expected and incredibly versatile. For many devices you don't need a cache battery (i.e., you can use the E-Werk to charge the device directly as you ride), and I'd expect that to be more efficient.

If you need a battery to store energy and output it via USB, you can get them very cheaply through Monoprice.com. Since batteries don't last forever, this would make for easy replacement when they stop holding much charge -- I suspect this will be an issue for me because I only intend to use this on tours, which I only do once a year or two, and I don't think lots of idle time is good for batteries.