Generally, you get what you pay for...
No not really, not in this case.
I mostly agree with Garbz here... Canon/Nikon would love for us to believe that their batteries are "super-special" and warrant the outrageous mark-up they charge for them. And, while 3rd-party batteries don't always exhibit the EXACT same level of quality as brand-name batteries, they generally get the job done just as well and can literally cost 10% to 33% the price.
Now, I don't shoot Canon, but I will relate my experiences with 3rd-party ENEL9-equivalent batteries as compared to my Nikon-made ENEL9s. The experiences probably apply equally to either manufacturer and the corresponding generic manufacturers.
The first generic ENEL9 that I purchased lasted for about a year. It held a charge just as well as the Nikon battery, lasted roughly as long as the Nikon battery, and overall functioned with almost no difference in performance. The biggest difference between the Nikon battery and the generic model was the price-point.
However, I did quickly discover a serious downfall of this particular generic model (I don't remember which brand it was, unfortunately). I went out to a place called Nonnewaug Falls in Woodbury, Connecticut during a SERIOUSLY cold morning last winter. Incidentally, I had my generic battery in my camera, freshly-charged. After hiking about a 1/2-mile through 12" to 16" of snow, I arrived at the falls at starting composing a few shots. It was so cold that the humidity from my breath was literally crystallizing on my tripod head! Anyhow, I shot about 30 photographs when, all of the sudden, I noticed that my battery was nearly dead. This is the sort of behavior I would expect out of alkaline batteries in cold weather... not lithium. Sure enough, my generic battery was dead after all of an hour since I left my car at the trail-head (1/2-mile hiking + 30 photographs).
If I had brought along only generic batteries of that brand, my shoot probably would've been ruined. Luckily, I had a Nikon-brand battery in my bag. I popped it in and proceeded to take another 60 or 70 shots. The battery showed no sign of power loss whatsoever. In comparison to the generic battery, it was basically unaffected by the cold. In fact, I moved on to other shooting locations in the area afterwards, and despite the temperatures, the Nikon battery lasted me the rest of the morning.
Once I returned home and let the generic battery warm up, it exhibited a disconcerting "bulge" in the casing which prevented it from even fitting properly in the camera. In short, it was ruined... presumably because it had literally frozen in a scenario when the Nikon battery had held up just fine. And, in case your wondering, the Nikon battery was
older than the generic.
You might think that I'd be bringing this story up as a reason to stick with Nikon batteries. But, in truth, outside of that excruciatingly cold morning, the generic held up just fine. It performed exceptionally well all Spring and Summer and earlier in the Winter. Considering that it cost me a mere fraction of what a Nikon battery would've cost me, it was well worth it. If I experienced brutally-cold conditions often here in Connecticut, I'd probably have no choice but to invest in Nikon-brand batteries, but days THAT cold are actually rather rare. And, frankly, I have only encountered this scenario once with only one generic brand of battery. There's not necessarily any reason to believe that all generic batteries would have that problem, nor can I say for sure that all batteries by that manufacturer will react the same way. And, furthermore, if I had incidentally stayed home that extremely cold morning, the generic battery may well have lasted a few more years! Who knows? I'm willing to deal with that tiny bit of uncertainty.
These days, generic batteries still comprise the bulk of my battery power. I own generics on about a 2:1 ratio to Nikon-brand batteries. And overall, the generics work great and I can own two or three for the cost of a single Nikon battery. I will admit, though, that I always try to take one Nikon-brand battery along, even if it just remains in my bag as a backup. So I suppose that I've personally come to the conclusion that Nikon batteries are more reliable than generic batteries. Still, though, the outrageous mark-up is practically robbery when you consider that generic batteries cost SO MUCH LESS and work almost just as well.
So... it really comes down to how much money you're willing to drop. If money is no object, then sure... you should get only the expensive brand-name batteries. However, if money is an issue... especially if you're busy saving up for a new lens or a new body... then consider that batteries contribute pretty much zilch to image quality. As long as you've got SOMETHING with which to power your camera, you're all set. Brand-name batteries ARE a tad bit better... but that "tad bit" will cost 3 times as much as nearly-comparable generic units.