I've had decent luck with aftermarket batteries with my Canon DSLRs and even with my smaller point and shoot cameras. Now with two Panasonic GH3 cameras, i've been looking to add a few extra batteries to the kit. I've been hesitating because the OEM batteries run about $80 dollars each (found here). So now i'm happy to see a few aftermarket batteries becoming available for the GH3. They claim to be decoded for the camera to read proper battery levels, currently run for about $20 each or sold cheaper in bundles with optional battery chargers. You can find these Aftermarket GH3 Batteries via eBay (click here).

Aftermarket Panasonic GH3 Batteries with Charger