All i've got to say, is that my friend had it on his Evo Shift, and before he couldn't get through half a day without plugging it in, and when he started using JD with aggressive, he was able to go almost 2 days straight.
Later, he had my other friend install it on her phone, which normally would last about a day on one charge, and set it to balanced, and she too was able to get about two days out of her phone.
Hearing this sorta news from them, made me want to tell my little brother, who has an htc EVO. He was ALWAYS complaining about battery life, and that he could barely make it through the morning without it being near death by the time he would head in for work (usually wakes up around 6am, and head to work at 1pm). So, as i said, i told him about it, and he installed it. idk what setting he has on it, but i know his complaining over his battery issue has stopped completely, and have noticed his phone isn't always on the charger at night.
Matter of fact, i just asked him how much he usually gets outa his battery, and his words were: "all god d*mn day!" So its bluntly obvious it works for him. and bluntly obvious that it works in general.
What I'd have to say about those who have used it, and never had it working, would be that they probably never had it actually turned ON, or, they are the type to constantly check their phone every few minutes, and so the effects of having the radios and such off had no real effect on them. That, or they check so much that by the time the radios are off, they are turning the phone back on and thus having some lag between the shut down and start up of the radios.
All in all, I know THREE people personally that get AMAZING battery life out of their phones when using Juice Defender. If it isn't working for you, it may actually be your phones fault for not being fully compatible with it, or its your own d*mn fault for not using it properly (which JD is really freaking easy to use).