It does exactly what it says on the tin.
I'd been having issues with the Macbook running stupidly hot and suffering constant fan noise, after leaving it running 24/7 for a few days. (ie. didn't even put it to sleep).
Given that I wasn't doing anything processor intensive, I shelled out for CoolBookController in order to shut it up. I've been running at a very cool 52 degrees-ish for a few days since I chopped the processor back to 1GHz. Battery life is up, too.
All-in-all, it's an essential purchase for anyone with a Core Duo laptop
- I just can't understand why Apple didn't let us throttle the processor in the energy saving panel.