This is one of two must have accessory for serious photographers (a remote release is the other). I purchased a battery grip for my prior camera and love it. This time I opted for Agfa rather than Canon simply because the Canon grip is seven times the price. That is a risk I decided to take this time.
This grip seems to do all that I need so far. It allows me to more than double my shoot times between recharges. It accepts one or two batteries which extends the battery time. With two batteries in the grip, I've shot intermittently for as long as 8 hours on one charge. In a fixed location, and two batteries you can keep one in the charger and one in the camera/grip while shooting long sessions or movies. It also comes with battery magazine which holds six AA batteries. Since I carry 12 rechargeable AA cells for the flash, it just increases the time I can shoot even farther.
The grip allows you to hold the camera vertically and maintain the horizontal grip you normally use. By adding shutter release, AE lock/FE lock, AF point selector buttons to what is usually the bottom of the battery pack they become perfectly located when you switch to vertical shooting. By rotating the camera to vertical the bottom becomes the right side and that is exactly where you are accustomed to have the controls. This means that instead of keeping your shutter hand above the body in the vertical shooting position, you can use the normal grip and keep the left hand on the lens rather than supporting the body of the camera. In the normal grip position you can keep the elbow anchored to your body for stability. This makes vertical shooting easier and enhances camera handling when shooting in the vertical.
I originally bought it for the battery life, but have thought and shot far more in vertical than I ever did before owning it.