This thread rekindled my interest in the subject and I've been doing a bit of research.
I read reports on a US site about the E-werks batteries (the cache) failing - one person had 3 out of 4 go. Also, reports of poor life of the battery; very quickly, it taking longer to charge and not lasting as long. This puts me off. The E-werks is not a cheap bit of kit.
I found the similar
Zzing. Looks good and around 2/3rds the price. Of course, it could suffer similar battery problems (I feel this is an inherent problem when charging a battery from a fluctuating/variable source like a dynamo).
There are several self build options (on which I am not keen as I have no ability in that respect!), and a number of cheaper options that do not incorporate a cache battery. I don't fancy those since I don't want to risk frying my GPS.
The PedalPower+ SIC looks good but isn't cheap either.
I still keep coming back to the conclusion that I'm content to use a simple battery extender (they are affordable, disposably affordable even) and AA batteries. It's rare for me to want extra GPS life so, given the price and considerations around the hub charger options, I think I'll stick with it. I do like gadgets though!