One of the biggest problems with non OEM batteries is in the manufacturing of the casing.
Quite often you have to be very careful and buy from a reputable 3rd party or you may find that may:
(1) not quite fit your camera right
(2) not fit snug in your charger
(3) not even charge correctly as the contacts arn't quite right
(4) not hold in a charger, especialy if you have one that plug straight into the wall socket and thus holds the battery vertical.
(5) not fully charge to stated capacity.
3rd Party makers such as Energiser, Delkin and Hahnel are all extremely reliable.
Olympus E-3 | Olympus E-1 | Olympus 14-54mm f2.8-3.5 | Olympus 50mm f2.0 Macro | Olympus 50-200mm f2.8-3.5 | Olympus FL-50R Flash