Appears most commenters aren't fully aware of how juice defender works.
It's not a task killer, totally different idea.
I love it, works great for me, if you don't have the paid version stick with balanced for now and make sure you don't find it inhibiting your regular phone usage.
I have the paid version so i have all the settings tweaked to my liking. I keep wifi on at all times, and it turns on automatically when i get near a know wifi point (uses cell towers NOT gps) which is really accurate.
I have it only turn on 3g when the screen is on and every 15 minutes for 30 seconds to see if any data is coming in, if it detects data then it stays on until it's don reviving/sending. This is the biggest thing that saves me power when i'm not home or near wifi. I used to lose major battery life when i was out and about, usually from low 3g reception or from moving, but now I can go all day with normal usage or longer with less aggressive usage.
Phone uses next to no battery at night because i have it set to disable all radios accept for cell between midnight and 8am, those can be automatically adjusted by a few hours by the app if it senses me using the phone during those times.
I'm continually impressed with how much extra life i get running JD and how unobtrusive it is to system performance and my regular phone usage.
It'd be nice to have data on at all times, and I can manage that, but I prefer the extra juice i get with JD over the convenience of up to the minute email updates.
If you run IM apps, then my setup or jd in general might not be your cup of tea, it will hold up any messages you receive for as long as you have data off.
I've yet to see any real negatives, I'm interested in what any actual users of JD have to say as far as negatives though.
I've never used the free version and never messed with the presets like balanced and aggressive, always used a custom setup tailored to how I use my phone.