When I bought my Canon 20D in April 2005, I bought an additional battery from B&H. The battery brand was "Power2000" (B&H house brand?) and listed a slightly higher overall capacity. The Power2000 BP-511/2 battery that arrived seemed to take a charge, but the camera would not power up with it at all--dead camera. I returned it to B&H and got another one just like it. I've been using the replacement for about a month, and it functions just fine. I have not noticed any difference between it and the Canon brand battery that came with the camera.
Due to the initial failure, I am inclined to buy Canon brand batteries in the future. But, the Power2000 battery was nearly $15 cheaper. YMMV.