I eventually uninstalled it. Initially, it "seemed" like I was getting better battery life. But after a few weeks of learning what to keep on/off I saw no difference between juice defender installed/uninstalled.
Bigger problem for me was that juice defender had a tendency of keeping the radio off too long when the phone came out of sleep. I always had to wait an extra 2-5 seconds for 3G to reconnect which eventually became very annoying. Once I had juice defender uninstall this wasn't a problem any more. Also, (not really sure if this is a juice defender issue), I stream BT audio to my car so I can listen to pandora through my car. Whenever the phone went to sleep the BT connection would be severed. I suppose the paid version of JD would let you configure this a little better. But after a few months I really don't see a need for juice defender.
I personally, believe in having as few apps as possible. Less updates, less potential issues with the OS and cross app issues and less nuisance.