The B & M e-werk has been around a while now and was originally what I was going to purchase. It's hellishly expensive (with the additioanl cache battery) but will charge plenty of different types of devices. The devices I have that need charging (smart phone, Kindle) only need USB, so when I saw the USB werk at a much lower price I ordered one from the Fatherland straight away.
It's a nice and small and fits neatly in one of the pockets of my bar bag. I've tested it out on a few short rides and it works a treat. The instructions say to give the cache battery 10 minutes at 15 km/h to ensure there's enough juice to charge constantly. I've found it only charges when I'm moving. I have no idea if this is normal or if my phone is a bit fussy but it doesn't really matter as once you are moving it charges away no problem*
I've deliberately run my phone down and then gone for a ride. It charges up just fine. In fact, I haven't charged my phone any other way in the last week!
So all in all I'm very pleased with it. It's going to be so much easier than finding a power point for a quick blast in cafes and bars etc
* Update: Today I tested the USB werk with my Portapow cache battery and it charged constantly - even when I was stationary, so clearly my phone is a little fussy...