A lot of Chinese batteries tend to be near-identical to name-brand batteries except for quality control. People underestimate the importance of QC, but manufacturing is only half the battle. Unless you're evaluating your quality to 1) improve your processes and 2) make sure defective products don't reach customers, you'll end up with wildly different reviews based on peoples' luck. Some people will have terrible experiences, and some will wonder what all the fuss is about.It's interesting that opinions on third party batteries tend to be either 1) I bought one that had an issue, therefore all of them are bad and 2) I bought one and it works well, therefore all of them are OK.
I wish some "real" brand name would make third party camera batteries, like Everready or similar, instead of mystery Chinese brands.
So, if the knock-off battery is 1/3rd the price of the Canon, you are basically gambling that there's a 1 in 3 chance that the battery you get will perform as well, or you're willing to accept that it may perform 1/3rd as well. The extra price you pay to Canon is largely for the guarantee that you don't have to mess around, and you can just be pretty sure you're getting something good in the first place.