I've always felt that shopping for camera batteries is about as exciting as buying tires for a station wagon. Yes, you need them both, but they cost more than you want to spend and are less than thrilling to use.
Since the manufacturer's batteries are pricy, it's tempting to save a few bucks using batteries made by other folks. So I decided to put this to the test with my Canon PowerShot S90 compact camera. When I purchased the second battery, I opted for the ProMaster NB6L Lithium at $24.95 instead of the Canon NB-6L Li-Ion Battery at $41.75.
The specs for both batteries are the same. And after a few months of alternating back and forth between the two, I haven't noticed any performance differences.
So my initial conclusion is: as long as the third party battery is of good quality, I don't see a reason to pay more for the manufacturer's brand. If you've tested this with other cameras, please post a comment with your findings.