Just bought the Zeikos ZE-NBG90, a non-Nikon product for my D90. My initial impression is very positive. Feels solid. Good grip material on it. Fits like a glove. Nothing about it looks like it was made by a third party manufacturer. We'll see how it goes over time, but initially "Two Thumbs Up."
I did a lot of googling since there are a lot of third party grips. I decided to go non-Nikon simply for price. Nikon grip is $140-$160. The Zeikos was $67.78 @ Amazon. I read reviews and this YouTube video finally helped me choose Zeikos.
Why the grip? When shooting sports I often shoot portrait (turning the camera 90 degrees). The grip puts another shutter release on the camera so when you rotate the camera your hand can stay in the same comfortable shooting position.
It also allows you to put two batteries in the camera for all-day shoots. I could see adding one of these grips for my D40. The D40 camera body is small and I could imagine how the grip would be more comfortable. (The D40 is still an amazing camera!)
Here are some pics of the grip on my D90: