I'm using the Nikon grip, which makes the camera well balanced in the hand with longer lenses. It's very comfortable in the upright (portrait) position for extended uses, if you have large hands the grip makes it much easier to hold and operate. Although the quality has improved on 3rd party grips I would never use one as a Meike once damaged the tripod bush on a D300 but that's just my view.
Incidentally they are giving free Nikon grips away with the D610 which fits the D750, this leads me to think they are not selling particularly well and my view is they will be withdrawn from sale soon.