Juice Defender Ultimate is one of the more popular batter life saving apps on the Market today. My biggest problem with apps like these is that often times the statements are mere posturing when just a few simple preventive steps by the user would do the same job.
Don't get me wrong, Juice Defender Ultimate is an incredibly intelligent app that allows for an insane amount of customization over the smallest aspects of the phone. I just feel the largest ways Juice Defender saves battery life are just as simple as turning off the wireless functions when not in use. There really is no hidden secret here, only have the GPS active when it is needed, don't enable WiFi when the phone is in a back pocket between classes, and keep the screen brightness at a reasonable level. This can all be done with a simple widget or notification widget like Widgetsoid 2.x, which allows for quick and easy toggling of various phone functions.
I am not usually craving phone battery, and my EVO 4G seems to do alright through the day, assuming I don't spend my hour between classes looking at cat videos on YouTube over Ohio State's WiFi. So the supposed 1.38x battery life bump I received was not as noticeable as the 4x claims made on Juice Defender Ultimate's Market page. I can see where this is coming from, however, users who do not know to disable specific wireless connections are sure to see a massive bump in battery life when their GPS isn't constantly activated.
Juice Defender is nice for when these functions have been accidentally left on, and a good option is to automatically disable all wireless data connections when the screen is off. This does conserve some battery, but a problem occurs upon unlocking and immediately utilizing an app that requires an internet connection because there is about a 15 second delay as the phone connects to a data network. The same applies for the option to keep data off unless specific apps are opened. It is a great theory, but in practice, there, again, is a long delay between when the app is opened and when the data connection is finally established.
One final problem I had with Juice Defender is that a few days after installing it my home screen really started to bog down and become sluggish. Individual apps did not seem to be affected other then in the initial launch. I do not have any hard evidence that Juice Defender is the red-handed culprit but I do know that as soon as I disabled it everything resumed normal function and I didn't experience any more problems.
Overall, Juice Defender Ultimate is best for those who have trouble regulating their connections by themselves or for those with older batteries and need every last percent. For most everyone else, however, Juice Defender Ultimate may just become a superfluous app that will benefit marginally with the chance of compromised performance.