Yeah, users' reports of its effectiveness seem to vary wildly.

My suspicion is that it depends heavily on the CPU governor [1] a device uses. Some will see that process running once a second, and keep the CPU at or near max frequency 100% of the time. Others will back down more quickly and you won't see much of a benefit, and still others were probably running at max frequency even before the change, so it wouldn't have much of an effect on them either.

[1]: http://androidforums.com/xperia-mini-all-things-root/513426-...