I agree with CharlieS. I have read too many tales of woe from users of aftermarket batteries. I do own one such battery that I bought before reading very much about them and I consider myself fortunate that it has given fairly good service for the past year or so. It claimed to have slightly higher capacity than the Nikon battery, but actually delivers between 10% to 15% *fewer* shots. At least it has not caused any other significant issues. I know that Nikon batteries cost more, but part of what you are paying for is the added quality control. For me, at least, that provides some peace of mind.
By the way, I never go out shooting without at least one backup battery. A DSLR with a dead/faulty battery is just an expensive doorstop.