This is a superb bit of kit for anyone using Canon flash guns, though not the 430EX II for some reason. It holds 8 AA batteries with another 4 in the flash itself and can keep up with a Canon 50D firing at 6.5 frames per second in short bursts; though I would not try this too often as the flash circuits are not built for it, but on the odd occassion it is fine. The pack can be mounted to the tripod bush of the camera, on a belt, or wrapped a round a lightstand via the velcro strap. I've used this heavily since November and it is still on the first lot of batteries, including doing a birthday party where I took over 150 shots on the night with flash. If you want quick recycle times and don't want to be replacing batteries too often then this is the product for you, and it's far cheaper than the Canon original.