I've gotten ebay batteries for several different things I own, and MOSTLY my experience has been good: eight batteries, as four sets of two, and only one individual was a dud. I didn't initially notice that often the ones listed are lower amp-hour ratings than the manufacturer's own, and one pair ran very short, so the last couple of times I've been more careful about reading the details. Still, I'm way ahead of where I'd have been buying Canon's own, for instance. Since lith-ion batteries normally don't have more than a year or two of life, and need to be replaced sooner rather than later, I figure I can buy a few duds and still come out OK in the very long run. Watch the shipping, though!
As O-Canis says, it sure is nice not to have to worry about running out of juice--I don't take my charger on trips anymore, for instance.