I have a third party battery grip for my D7000 and since it only cost about £30, I would consider it a steal. Given that the Nikon battery grip for the D7000 is selling for between £150 and £200, I find it hard to see why anyone would opt for the more expensive option. I’m sure some people buy Nikon/Canon accessories and nothing else but in this case I feel it is simply unjustified.
There might be some differences in the construction and I believe the Nikon one is more solid but considering that the third party one is around 20% of the price, if it breaks I can buy another one and still be in the position of saving a lot of money. Feature wise, it is identical to the MB-D11 produced by Nikon, very handy- makes the camera feel more substantial and is useful particularly when shooting portrait, plus it was very cheap! I think the only significant differences are that some (early ones) have the dials reversed (relative to the body), the newer ones like mine don’t have this problem however.
I would recommend one to anyone with a DSLR with battery grips available (i.e. the majority of modern DSLRs). The one I purchased was from Amazon, I believe it is listed as ‘Neewer’, although it is actually the ‘Meike’ grip for which there is a lot of information available online.