So, I've had this app for a while now, so I thought I would reserve my judgement until I've had enough time to test it. Well, it WORKS! JuiceDefender is free on the marketplace and it has the following options:
1. Disable 3G APN/WiFi/MMS when the screen's off (This is what helps the most)
2. Disable 3G APN/WiFi/MMS when battery is low.
3. Disable WiFi when far away from known WiFi networks.
4. Enable 3G apn/WiFi for 1/2/3/5/8 mins every 5m/15m/30m/1hr/2h (Very useful if you want to download e-mails and sync when the screen is off)
5. Night schedule - turn off everything, basically airplane mode, when you sleep. Also has an option to silence.
There's also an option to place a widget on your homescreen with options to quickly disable, and enable the app and also shows an estimate of how much battery life the app saved for you.
While JuiceDefender is free, UltimateJuice is not. The only difference that I noticed between the two was that UltimateJuice had the option to turn off WiFi too, while JD was limited to 3G APN and MMS. I believe the app costs around $2.99, and the dev is VERY responsive to suggestions and issues, so if you like JD buy UltimateJuice to support the dev!
Note that it says x1.04 on my widget because I had just reflashed a new ROM!
More info here: Latedroid
PS: I'm in no way affiliated to the dev, just an extremely satisfied user!