I bought a genuine Nikon one for my D700, got a mint, unused one off ebay. I think it was unwanted as Nikon had an offer on where you got them free with all D700's for a while, and clearly the owner didn't want it. Think I paid £120 - 130 and am very pleased with it. Be warned though - it is metal and adds weight and bulk to an already big and heavy camera, especially with a big lens like a 24-70 on.
Does it do anything over and above a 3rd party one? No probably not, but if I'm using a top end Nikon camera and lenses, it didn't feel right to be using a 3rd party grip on it.