I don't think a generic battery will damage your camera but I would only buy a genuine Nikon battery. However if you must buy a generic be careful where you buy it from and if it's a respected battery maker. I would not buy from an ebay seller. Instead a reputable dealer like B&H is much preferred.
As Nikon comes out with new cameras and new batteries they are putting in chips which are recognized by the camera as a genuine Nikon or not. I haven't checked on the D800 or D600 camera batteries but I know that a generic battery will not work at all in the D3200. While this is not the case with the D700 battery it is something to think about in the future.