This app is well worth the tiny price. I have used it for almost a year now with a 17" Macbook pro (core 2 duo). The default settings left it running fairly hot, but after spending 20 minutes figuring out the minimum voltages needed per frequency I had some settings that have been totally stable.
When you're on battery you get much better performance and use less energy, by setting the voltage to the minimum but the frequency to the max that voltage can power (for me this was almost double the CPUs normal minimum).
and for desktop work you can use the max CPU speed with a big cut in power, meaning less heat and nose from fans.
In case it helps someone out, my settings turned out to be; 1837MHz-0.95V, 2004Mhz-0.9625V, 2171Mhz-1V, 2338Mhz-1.05V.