tl;dr: You can (and should) pick up one of these things if you have a D90 or D80, and shoot vertically or need extra juice for the camera. The Nikon MB-D80/OEM model is a bit more solid, but you can't beat the Phottix's price!
Take a look at what's hanging on the neck of almost any professional photojournalist, wedding photographer, or camera guy—you'll almost always see the tank-like and bulky body of a Nikon D1/2/3/X-series or Canon 1D-series SLR. The Phottix MB-D80 takes your D90 (in my case—it also works on the D80, though) and gives it three of the tank-like bodies' functions:
- A vertical shutter release/controls: When shooting portraits, this is immensely helpful.
- Double the battery life: You can stick an extra Nikon EN-EL3e battery in the grip (bringing your total to 2 batt's!).
- Look like a serious photographer: 'Nuff said.
The battery grip performs great, it can take a beating, and the shutter release and controls feel almost as good as using the controls on the camera body itself!