Having had my Canon 40D for several months now I decided to buy a battery grip.
The grip gives photographer a vertical/portrait grip with its associated controls and shutter release. And it gives you room for two BP-511 batteriess.
I don't feel comfortable paying well over hundred Euro for a piece of plastic so I decided to give it a try with a 3rd-party device instead of genuine Canon BG-E2 (or BG-E2N).
Searching on eBay I found a few options and finally selected one offered by HK Supplies (Hong-Kong eBay vendor, with eBay username etefore. I bought some accessories from that vendor in the past and was quite satisfied with the quality, price and speedy delivery). Their offer also included 2 Phottix bateries (Canon BP-511A equivalent).
My order arrived in about 1 week and my first note is that it was nicely packed.
You can see the content of the box on the picture above. I took one of the batteries out of it's little red box; and the grip is still in the larger box.
Besides the grip itself there are 2 battery containers - one for 2x BP511 and another for 6x AA-size batteries. Comparing this grip with the photos of genuine BG-E2 they appear virtually identical. The C40DS grip is made quite well - no cheap feeling, strong, attached without any problems to my camera, and the buttons feel good. The tripod socket also feels sturdy. No flimsy parts, good solid feeling.
Talking about batteries I should note that I used some no-name ones in the past and never got any problems. I had one battery out of 4 suddenly died after 1.5 years of use but with the price under 10 Euro per battery I believe it's much better deal than the brand-name batteries.
The new Phottix batteries that arrived with the grip are marked as 1600mAh, slightly more than the Canons 1390mAh so they should perform better (at least on paper). I don't have time or desire to do any formal testing/measure, but my feeling is that the batteries aren't any worse than the original ones. They look well made from outside, they fit without any problems to camera, charger and to the grip; and camera works just fine. Again, I didn't measure the number of shots I can make with substitute battery but it feels quite similar to the original.
I would definitely recommend both the C40DS grip and Phottix batteries to anyone thinking of increasing their shooting capacity.
Remark: Please note that there are other battery grips on the market that are marked "BG-E2 equivalent". Some of them don't have the index and focus point buttons; and some reviews suggest that some of the grips are flimsy - so be careful which one you select!