OK, on my D90 I have Vello grip and it seems fine... I think if one purchases the grip from a reliable company the results are better. In the past I have made some purchases from less than reliable companies, those without brick and mortar stores, and the results were poor. Cost me about a grand... Some of the Chinese goods may be made on the same line as a more brand name item. There are many areas in merchandising where an time is manufactured and a generic name placed on ti along with a price, then at some point, the brand of the item is changed and the price may double. Some manufacturers make there own "generic" drugs and this is done to cover the market so as to avoid another manufacturer taking part of the market.
It could be very possible the grip is made on the same line as the Nikon grip, although not highly probable.
So, sometimes, the "cheap" grip is actually cheap only in price. But, the final retailer will be the one who is the critical piece.