The Canon BG-E11 Battery Grip is the battery grip designed for the Canon EOS 5D Mark III DSLR. All of Canon's Battery Grips are fundamentally similar to each other in design and functionality, and once again, the BG-E11 includes the basics.
DSLRs without a built-in vertical grip (all non-1-Series bodies at this time) have the advantage of being compact and light, but these bodies are not comfortable to shoot with in portrait/vertical orientation - especially for long periods of time. An accessory battery grip solves this problem by adding a vertical grip complete with shutter release and other relevant buttons. And, the grips allow a second battery to be used for double the battery life.
Being easily removable, battery grips allow the DSLR's other advantages to remain available.
To install the BG-E11, the LP-E6 Battery is removed from the 5D III and the battery door is detached. The 5D III provides a small spring-loaded latch release switch on the battery door to make its removal especially easy. The battery door then attaches to (installs into) the grip for storage (snapping into place over a small instruction diagram).
The BG-E11 slides up into the empty battery compartment and locks into place via the tripod socket screw on the BG-E11. The BG-E11 provides its own tripod adapter threads - this functionality is not lost.
The BG-E11 holds two Canon LP-E6 Batteries that deliver double the number of shots or video time from the DSLR. Alternatively, use 6 readily available AA batteries in the included Canon BGM-E11A Battery Magazine (a small nylon bag is now provided to store the unused battery magazine in). The BG-E11 works fine with only one LP-E6 battery - a second LP-E6 battery is not required.
Don't forget to turn the grip's power switch on.
The Canon BG-E11 Battery Grip is very nicely built - it integrates especially well to the 5D III body with little flex. As you might expect, the amount of BG-E11 flex is more than with the integrated 1-series grips (with minor tripod use implications), but Canon has done a nice job of meshing the grip to the camera.
Like the BG-E9, the batteries load from the end of the BG-E11 grip and orient in line with the grip allow a less-squared and notably-better ergonomic design than the previous battery grips with their rear-loading batteries. Still, the BG-E11 grip is moderately bulky - bulkier than the built-in 5D III grip. The normal grip becomes even nicer with the BG-E11 attached - providing more landscape for you hand to grip the camera with.
In use, the BG-E11 makes portrait/vertical shooting much easier and more comfortable. I can feel the fatigue difference in my arms and shoulder after using a battery grip for even a moderately long shoot. And, I can hold a gripped camera steadier in vertical orientation - my right arm can be stabilized against my body instead of being held overhead.
New for a Canon battery grip is the BG-E11's inclusion of an additional Multicontroller/joystick positioned for vertical use. The new Multicontroller is positioned just slightly less conveniently than the 5D III's built-in Multicontroller, but it is far easier to use in vertical orientation than the 5D III's Multicontroller. This position is likely as-good-as-possible without changing the overall size of the grip - which would not be a good idea.
The BG-E11's large thumb and shutter release overhangs provide an especially sure handgrip. If you are active while shooting, the more-sure grip is going to be especially welcomed.
Overall, I think the Canon BG-E11 Battery Grip is very useful - it is definitely a nice-to-have accessory. The price of this grip will limit its adoption to serious photographers only.