I agree with Niko. I have generally bought several spare batteries for each of the Canon cameras I've had (S60, G12, S95, 10D, 5D, and 5D2), and the cheap knockoffs have been as reliable as the genuine Canon ones. Some have slightly lower capacity than the Canons and others slightly more. The only one that has ever failed for me was one of the Canons for my 10D.
I generally get the cheapest ones that B&H sells, 3 or 4 at a time so I don't worry if one quits. I always carry at least two spares with me, and more than that in cold weather.
Eric