I'm not sure why people are bashing automation and convenience. "Yeah, you can just do that manually." Well, of course. But why should I? In fact, that's why I love Tasker, because it automates things for me instead of my having to do things manually.
Just to clarify a few points here:
- Juice Defender is not a task killer. It turns off data when your screen is off, and then turns the data back on when your screen turns on.
- If you are constantly turning your screen on and off all the time, the battery savings will be non-existent. In fact, using something like Juice Defender may actually hurt your battery life with that usage pattern. If, however, you use your phone for a short stretch of time, and then you leave the screen off for a long stretch of time, you'll see major battery savings because that whole time your screen is off, your data is also off.
- Data toggling can be a tricky thing. I noticed that Juice Defender was great on my MyTouch 4G, which could successfully toggle data on and off with no problems. However, on my Verizon Galaxy Nexus, data toggling was a bit finicky. Sometimes data would come on. Sometimes it wouldn't. Sometimes I'd have to switch to airplane mode and back for data to come back on again successfully. Your mileage may vary.
- As someone else noted, Juice Defender hasn't been updated on a long time. It still performs the same basic function (turn data off when screen is off, and turn data on when screen is on), but since development has stopped, if new bugs arise because of new device configurations or new versions of Android, no one is going to fix those.