One of the easiest choices is to buy brand name batteries. From batch to batch the manufacturer takes quality and performance very seriously. You know and I know that in general there will be no lemons. Generic batteries can be made by any number of manufacturers and they all take on the challenge with different perspectives. And as a result if you were to buy from a e-retailer www.dealextreme.com (which has been selling generic batteries from many manufacturers for years) you'll note that customers leave reviews indicating that one generic marque is better than another. One marque will hold almost as much charge as a Canon/Nikon marque for example.

But because generic battery manufacturers often just copy-cat the design and don't really take special care in being faithful to copying everything, they'll miss something that is non obvious. For example there are several cold weather expeditions to Antartica, mountaineering, and ocean faring where photogs discover generic brands to give out all too quickly, rendering the whole trouble of carrying them moot. They all chimed in wishing they had packed genuine brands. So you can imagine a generic battery maker using thin plastic shields without weather sealing because they could shave off a couple of cents and improve profits--not knowing what they are about to do to some of your most important photographic adventures.

Batteries are also intimately connected to the electronics in your camera, you don't want to be in an uneviable situation where Canon or Nikon disclaim their responsibility to repair your camera because they've discovered that the electronics may be busted due to a generic vertical grip, charger, or battery. That could be quite costly.

But do what you must, and do it smartly. For example, you might have some bust up out of warranty camera you bought at a garage sale and you just need to find some el cheapo battery to go with it. So that you can take out the IR screen of the old busted camera's sensor and do some art projects. No harms done and your wallet won't feel all too much lighter.