Others say...

"Canon BG-E4 Battery grip for EOS 5D Digital SLR Camera"
Ergonomically its superb for vertically composed shooting. Feels solid and sturdy. For travel in remote areas where power outlets are scarce the 6 AA battery option addresses that issue, this is the main reason I purchased the grip. As far as ruggedness goes, I'm taking it back to Alaska this summer and down to Nicaragua so I'll update this post after I return. The camera feels significantly more sturdy and "balanced" with the grip attached when used with the 24-70mm f2.8L and 70-200mm F2.8. For situations where discretion is required, the extra size can be initimidating for you potential subjects. The price seems a little steep.

"necessary, but it is Canon and you pay for the name"
I think the grip is a necessity. It changes the whole feel and balance of the camera for the better. You also have the advantage of running off of two batteries with twice the shoot time without changing. Over $200 for a glorified battery holder with 50 cents worth of electronics is tough to accept. There are Chinese knock-offs that are half the price but I guess if you are going to spend over 2 grand for the 5D camera another couple of hundred for the Canon name (quality?) should not even be a concern..

"Must have accessory - marginal quality"
This battery grip is absolutely necessary for any application. It functions flawlessly.

This is my second BG-E4 and without a doubt, the materials quality of the unit is marginal at best. My camera was on a tripod and took a short spill and the minor impact cracked the grip casing in two places. Bad job on my part for being careless, however the grip should have survived the short-distance, minor impact.


"5d Battery extension Grip"
This turned out to be invaluable during my trip to India. I did not have to worry about recharging my batteries. During the 15days in extreme heat it worked great at the end of my journey I replaced the batteries, Shot close to 2000 photos. Also the controls being moved to grip was especailly handy for the shots taken in portrait mode.

"Useful"
Although I don't find myself using this too often, I do not regret buying it. I will use it at my next wedding. I read other reviews about it feeling cheap and did not feel the same - it feels fine and works great - it just adds extra weight. I have not yet expired one single battery, so I really don't have a use to store 2 in there.