Luis - a.k.a. Magestyk wrote a review of Petzl ACCU Core Rechargable Headlamp Battery on May 8, 2011
This battery pack has potential if they improve it(Larger mAh/Slow Discharge), but right now it's not worth the cost.
Let me explain. If you going on an Indiana Jones type expedition or traveling to some place that has no batteries/electricity around. Then 'Yes' this pack is worth paying/getting. The USB charging cable integration is fantastic. I have a solar(Power Monkey Explorer) charger and it works great with this pack.
Now with that said if you going to be around electricity, then it's not worth paying for.
It's better and cheaper buying AAA Rechargeable batteries w/charger.
I have a 4pack of 1000mAh and the performance I get is far better than this pack. However when I go for long hikes/trips I take the solar charge and this battery pack with me.