There has been many reports that enabling the numeric battery feature on the iPhone actually kills the battery life 
quicker. Some people have reportedly found a massive increase in battery life after turning off this simple feature. Especially those with jailbroken phones who use SBSettings to turn on the feature.
Some people oriignally thought the battery died quicker, but thought it was a psychological thing and that you thought it was as you could see the percentage go down. However I’ve now turned off the numeric battery for good and hope I get better battery life as currently it’s not lasting a whole day with light use.
Sam's Blog 
Just wanted to add another positive review to encourage customer investigation of this product. Their website is very communal, their brand story is interesting and this product really exceeded my expectations. It is easy to use and convenient to carry, has wonderful design relative to phone, and really helps me feel more secure when I’m traveling or heavily using my I-phone. The 3G JUICE really increases the value of my phone since the battery is unfortunately its biggest liability, and I don’t think I could live without it now. Highly recommend!
http://charger-battery.blogspot.com/2009/08/3gjuice-black-20.html