OEM batteries are generally way overpriced. They're all just batteries. That's not to say there's not a large number of rip-off 3rd party batteries out there. I've purchased a number of batteries from Sterlingtek and they've equaled or out performed their OEM counterparts. I've also purchased a large number of cheap-o-batteries off Ebay and generally have had very good luck. Some last longer than others or hold charge better, others don't. If you want to play it safe, but still not pay OEM prices, buy 3rd party batteries from reputable dealers like B&H or Adorama. Sometimes you can find Amazon reviews on batteries they sell too.
My Fuji X100 eats batteries for lunch so I like to keep around 3 spares in my pocket. It would have cost me over $100 for the OEM's, but I paid $20 for 4 off brand and they last about as long as the OEM batteries and have been for 2 years so far.