I have an aftermarket zeikos grip for my d90 and a nikon grip for my d700. I'm going to compare the two assuming that the d700 grip (mb-d10) and the d90 grip (mb-d80) are of the same quality. I don't have any experience with the mb-d80 so if its quality is not up to par with that of the mb-d10 someone chime in.
The nikon grip is magnificent. Never had a problem at all. Feels solid. Controls work well. It feels great in your hand. Solid.
The zeikos grip works great as well. The controls work great too. However, it does not feel great in your hand. The fit is fine but it feels very plasticy to me. The finish on it is close to that of the camera but still different. I can touch the camera with my eyes closed and tell whether or not i'm touching the body or the grip. The battery door is a bit flimsy. Lastly, the most important part of it that gets me is that the grip doesn't fit completely snug onto the camera body. Tightened all the way there is still a little play. This causes the grip to not feel as firmly attached to the camera as nikon's grip and also the little movement it can make can move around the contacting points the grip has to the camera and can cause inaccurate battery readings. On a budget, this grip would work great, great pricing in comparison and it will work well.
Bottom line: Even though the zeikos is much much cheaper, if I were to buy again,I would get a nikon branded one.