The difference in price between the Canon LP-E4 battery back and the equivalent model from Energizer is significant. What, if any, are the main disadvantages to purchasing the cheaper model as opposed to the Canon brand version?
It looks like the Energizer compatible offers less longevity: 800-970mAh (depends on the exact subtype of the Energizer one) vs. the official 1080mAh of the Canon brand. I could not find any information about the Energizer providing any of the data feedback, such as power level and the like. I did find several sellers that made a key point out about how to go about the first charge, and some caveats with the first charge.
When it comes to Canon batteries, your paying for two things. Obviously, your paying for the Canon name, to a degree. Additionally, you get the Canon guarantee of quality and longevity, full data feedback, and no quirks or restrictions on charging. Having used a few off-brand compatible battery replacements for my XSi myself, I can say one thing with certainty:
Canon batteries last FAR longer than most compatible replacements. On a single fresh charge, my Canon battery would usually last at least two times (2x) longer than any one of my off-brand replacements. When used in extremely cold conditions, the Canon also outperformed the off-brands, although to a lesser degree.