Heard this mentioned a few times here, does it work?
I tried it when I first got my Desire but I wasn't impressed. Too buggy and a lot of the things it does you can just do manually.
The best thing to do is add the "Power Control" widget to the homescreen and with the press of a button you can turn off things like Wifi, BT, Auto updates, screen brightness and so on all of which have a big effect on battery life.
http://www.lifeofandroid.com/app_review_detail/juicedefender/