I've did have the app for a few weeks.. for some reason it kept crashing when I was changing a few settings. I reinstalled it and finally got it to work a few days later.
I never really found much difference. The Redux rom doesn't seem to use that much power, even after tweaking the CPU to run slightly faster. I tend to manually change my Wi-fi, 3G settings and brightness if I need to save power for the rest of the day. The only app that uses data in the background is the weatherbug app that updates itself automatically every 2 hours. Gmail I rarely use on my phone due to the amount of pointless emails advertising stuff I don't need.
Even when I play with it heavily with wi-fi, GPS I'll still have about 40% without the Juice Defender app. From my short experience with it I only found that it turned 3G, Wi-Fi on and off, something that takes seconds. I suppose it's a good app if you need to be constantly updating stuff like Gmail, facebook, twitter while you leave your phone unattended as the app will automatically turn data on and off for short durations to allow for updates.
I suppose it's down to what you do with your phone. If you have a need to have your phone updating every 10 minutes then perhaps download it. I don't so I've since uninstalled it.