unfortunately, this utility doesn't seem to make any difference unless you're maxing the processor out at a given setting. if i set it to 12.5% and do word processing, i get basically the same battery life as if it was at 100%. if i'm running SETI or playing a game, which uses the CPU at 100% all the time, then turning it down makes a difference, but of course then you're really feeling the performance penalty. all it does is keep your computer from doing things quickly, which of course increases battery life (but also means it takes longer to do things).
for tasks that are purely CPU limited, you're probably better off running at 100% because then you're not wasting power running the drives, screen, etc while the CPU plods along. the only thing i see it as being useful for is if you're gaming and are willing to accept a lower framerate in return for more play-time (and it works pretty well for this, as long as you've got framerate to spare).