I am very choosy about what goes in my camera, because I am paranoid about damage, but these batteries (which I bought on a wing and a prayer) have been wonderful.
I have been using them for nearly a year now with my
Canon EOS 40D (also reviewed), and they have never let me down. Like the genuine Canon BP-511A battery, these are Li-ion batteries. They do not discharge rapidly when in storage, they hold more charge than the genuine Canon battery (1600mAh compared to 1390mAh), and the cycle time must be the same since I have noticed no difference in camera functions, even when really pushing my 40D writing high speed sequences of large RAW and large fine JPEG to the memory card.
I use two of these which are usually together in my
Canon BG-E2N Battery Grip (also reviewed). Normally I will use the Canon battery on its own in the camera body, or these two when using the grip, but I always keep them with me in case I need additional power. I have found that when used with the grip, one of the batteries will usually discharge more than the other while being used. I am not sure if this is due to the batteries being slightly different, or the design of the BG-E2N, but it does not appear to have any effect on my photography so I am quite happy.
My advice is to save some £££ by buying these instead of the horrifically over-priced Canon BP-511A. The money you save can go into your L-series lens savings account ;)