I like BatteryLife. Tells me everything I need to know, and is decently configurable. I've never had battery life issues while running it, but that isn't scientifically recorded or tested data, so take that with a grain of salt.
I've used gauge battery widget on the past and loved it. But recently I have been using Battery Status and i am really liking it. It gives you several icons to choose from for the status bar and lots of info in the pull down. IMO you can't go wrong with either.
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