I have two genuine Nikon MB-D11 battery grips, one bought new in 2010 when I bought my first D7000, and the other bought used (1/3rd of the cost of the original) for my second D7000.
Whilst the initial outlay seems a bit steep, the build quality is excellent (magnesium alloy shell) and mine has survived many knocks and a few drops but has continued to perform faultlessly. I never really considered the third party options - sure they could have saved me a small fortune, but the Nikon one is well built and guaranteed to function perfectly as it was designed by Nikon for the D7000.
Do the 3rd party providers guarantee perfect compatibility and warranty against any issues with the camera that may result from the use of their products?
Their offerings will always be cheaper - after all, they are reverse engineering and copying an existing product. It's far cheaper and easier to do that than it is to design, develop, prototype, test and refine a product from scratch.