PIXEL External Flash Battery Pack for Canon 580EXII/580EX/550EX/MR-14EX/MT-24EX (4*AA/8*AA)

Pros: -It holds lot of batteries.
-the cartridge is sturdy
-2 statuslights
-very well build, no squeeky plastic etc.
-large and looks cool :)
-Fits with a small "click" on my 580EX II.
-weathersealed connector (not the pack).
-fast release switch for the cartridge.
-really gives a good jolt for the 580EX II.
-it clicks very gentle, sounds kinda cool.

Cons: -the batterypack is too big for my Canon EOS 50D, even for the batterygrip.
I dont want to know how stupid it will look on a 350D/400D/450D/500D/550D etc.
-although the cartridge is sturdy, it takes time to swap the batteries, when you use the flash MANY MANY times, and you need to swap batteries in the middle of something, you are screwed.
-there is only 1 screw to attach it to your camera, and you can turn the batterypack around, even if you tightened the screw very hard.

Other: As a professional photographer I like the pack very much.
I did not buy the Canon original since this pack allows for easy cartridge release and is low-lost.
I shot many weddings without the pack, but surely would like to experience one with it attached :)

Pros: Lightweight -very little more than the weight of the batteries alone.
Decent length of cable from pack to flash - no restrictions on movement even if the pack is on your belt and the flash held above your head at arm's length.
Smooth contours which don't snag on clothing when rushing a shot.
Plug to flash seats extremely firmly and seals with rubber shroud.
Battery fitment very positive - two individual banks, well seperated physically (obverse and reverse of tray).
Batteries click very positively in place, no sloppy connections - tray contacts positively to pack when inserted.
Recycles the flash extremely rapidly - set to quarter power, the pack will keep up with an 8fps camera (but don't do it too much and ruin your flash!)

Cons: Well, this is a lot harder than listing the 'Pros'!
Far from vital, but an on/off switch would be reassuring instead of relying solely on unplugging from the flash.
The catch for the battery tray is slightly 'clicky' to lock in place - something which a tiny amount of wear will probably cure.
The belt loop on the case is a little bit tight for a 2" belt (now that's getting really picky!)
Two press-studs to secure the bottom flap on the case would be more reassuring, especially as the current one is plastic.
That's it - and they really took some thought.

Other: Offers enhanced performance over camera makers external flash pack at a fraction of the cost.
Better than the camera maker's offering; cheaper than the camera maker's offering - if you actually NEED an additional power source for your flash then this is the one I'd recommend.
Forty years professional experience have made me very critical and this product fulfilled every single one of my requirements.