There's a broad spectrum of aftermarket batteries, some perfectly fine and others, not so fine. I stick with 3rd party batteries sold by major retailers who have tried them out, themselves (e.g., B&H, Adorama, etc.). It's worth the hunt, as still seem to run as low as a quarter the OEM price and work fine.
p.s. The newest "chipped" batteries are tougher to clone, but it can be done. Just got two batteries and a charger for my E-M5 for half the price of the still unavailable OEM cell. The battery meter works, so somebody reverse-engineered the thing.