The Canon BG-E6 battery grip serves a couple of different purposes for the 5D Mark II. As the name implies, it holds batteries – either two LP-E6 lithium-ion batteries or 6 AA batteries in the included battery holder. Likewise, the BG-E6 serves as a grip for handling the camera in portrait orientation, featuring controls for the shutter release, scroll wheel, and focus point adjustments, among others.
The BG-E6 is a nice accessory to have; however, it runs $250. For some, that’s a little steep for a battery grip. Third party grips are much more affordable at less than $100, but are they worth the compromise? This question comes up on forums quite a bit and finds its way in my inbox now and then.
[UPDATE: I've supplemented this review with a review of the Vello BG-C2 and BG-C4 grips for the Canon 5D Mark II and 7D.]