I registered CoolBook, fully prepared to battle with the author to get my money back if it didn't do what it claimed.
Fortunately, it seems to
There's two parts to CoolBook:
1) Changing the speed. I use this when I play Civ IV, as civ IV chews up every free cycle, making my fans noooisy. I limit my CPU to 1002 MHz when playing, and all is good (and still fast!). I suspect what I'm actually doing is CPU-limiting my machine so the GPU doesn't overheat - see below.
2) Undervolting the CPU. I have a MBP C2D, and the original reason I wanted this app was because I play WoW. Again, my fans were noisy when playing. At my native 2171 MHz, the normal voltage is 1.1375V. After testing, I can safely run at 0.975V. I haven't noticed longer battery life (mostly plugged in), but my CPU temperatures noticably dropped (I forget my tests, but it was at least 5ºC) and fans ramp up a lot less. I'm pretty sure fan ramping is now down to the GPU getting hot, so am waiting for Aticellerator so I can underclock that too
Undervolting the CPU I leave on all the time, as I only see beneficial effects from that. Limiting the CPU speed I use when I get irritated by fan noise