I bought two maximal power batteries last spring, from Amazon. The camera recognizes them and allows you to register them, just like the Canon originals. Also, they have a fairly good warranty (3 years).
At the time I bought them, they were about $24 each. I recently checked and they are down to about $16.
Initially, I didn't notice any difference, but within a month or so, I noticed they only indicated three bars, in the camera, after a full charge. After six months a full charge only showed two bars and they would run down after about 50 shots. Also, the charging time became very short like 20 to 30 minutes. I tried leaving them in the charger overnight, per the instructions on the back of the manufactures package, but that made no difference.
I number each of my batteries and rotate through them, in order. That way I know they get used semi-equally and that one doesn't end up in the bottom of the bag for months on end.
Recently, I sent the bad batteries back for warranty replacement. Maximal Power was easy to deal with. I called them and explained the situation and they emailed me an RMA. Other than the inconvenience of having to return the batteries and the $5 or $6 shipping, it wasn't bad. I sent them to a location in California and had the replacements within two weeks from the day I mailed the bad batteries.
It's possible that my original batteries were from a bad batch. I've only had the replacements for about a month and so far the camera is reporting four bars, after each charge, so I can't say if the replacements are better or not.
My original Canon batteries are about a year old and still report four bars after a full charge.
If the replacement batteries hold up, I'll buy additional Maximal Power batteries, without hesitation. If they don't, I'll probably buy additional Canon batteries. Sure, the Canon batteries cost more up front, but the convenience and additional life would be worth it to me. Time will tell.