What does it do?
Unlike many other battery saving apps, Juice Defender has the ability to turn on or off apps that can add up to a big difference. When the screen is asleep, Juice Defender can turn off all data connections, including 3G and WiFi. The app can even do things like turn off data usage entirely at night or disable data and WiFi services if the battery dips below 20% power.
How does it work?
Juice Defender is a fairly minimalistic app, with a dark background...but it's all about what the app can do, not about the way it looks. Right off the bat, the app attempts to save you some juice as part of its "balanced" profile. You can also choose an "aggressive" profile; this is the one that'll help a dying battery conserve power by cutting all but the most essential functions. The "extreme" profile, meanwhile, disables all connectivity by default, saving a lot of juice. You can easily enable connectivity again by simply pulling down on the notification drawer and tapping on the Juice Defender's entry.
But where the Juice Defender really shines is in the settings tab. Here you can choose which apps you want to try to connect to the internet while you're using them. You can also periodically enable connectivity to let background data sync occur for apps like Twitter or Facebook, so you don't miss out while still conserving battery life. Not only that, Juice Defender has a setting that allows it to memorize where you are, so that your phone can attempt to connect to WiFi only when you're in certain locations where you're actually able to connect. This is incredibly useful for people who have a daily routine where they use the WiFi at home and at work, but have no need for this feature during transportation between the two places. Finally, this is where you can select how long you'd like the app to turn off data usage at night.
For all the different profiles, a different icon rests at the top of the screen to let you know how the app is conserving energy. A shield with a blue dot means some radios / interfaces are disabled, a shield with a red dot means all radios/ interfaces are disabled and a shield with a green dot means force-enable app is running.
How well does it work?
Over the past week, Juice Defender nearly doubled my battery life. I was able to find a customization that easily worked for me, forcing the phone to only access WiFi in specific places, only connect to the internet at certain intervals and only use WiFi or 3G during the day, not at night. My phone easily lasted through the day, without getting below 20% until around 10pm at night. I recommend this app heartily!